Femtosecond Time-Resolved Photoelectron Spectroscopy in the

Transcrição

Femtosecond Time-Resolved
Photoelectron Spectroscopy
in the
Extreme Ultraviolet Region
Dissertation
zur Erlangung des Doktorgrades
der Fakultät für Physik
der Universität Bielefeld
vorgelegt von
Peter Šiffalovič
aus Bratislava, Slowakei
März 2002
Betreuer: Prof. Dr. U. Heinzmann
Priv. Doz. Dr. M. Drescher
Gutachter: Prof. Dr. U. Heinzmann
Priv. Doz. Dr. F. Stienkemeier
Prof. Dr. W. Wurth
Tag der Disputation: 10.06.2002
Teile der Arbeit sind veröffentlicht in:
•
Laser-based apparatus for extended ultraviolet femtosecond time-resolved
photoemission spectroscopy
P. Siffalovic, M. Drescher, M. Spieweck, T. Wiesenthal, Y. C. Lim, R. Weidner, A.
Elizarov, and U. Heinzmann, Rev. Sci. Instrum. 72, 30 (2001)
•
Application of monochromatized high harmonic EUV radiation for inner-shell
photoelectron spectroscopy
M. Drescher, P. Siffalovic, M. Spieweck, and U. Heinzmann, J. Electron. Spec. Relat.
Phenom. (to be published)
Contents
Contents
Acknowledgements __________________________________________________________1
Introduction________________________________________________________________4
Theoretical Background ______________________________________________________9
2.1 Light–Solid Interaction _________________________________________________9
2.1.1 Photoexcitation and Photoemission _____________________________________9
2.1.2 Relaxation Mechanisms in Metals _____________________________________13
2.1.3 Relaxation Mechanisms in Semiconductors ______________________________19
2.2 Pump-Probe Technique________________________________________________26
2.2.1 Principle of Pump-Probe Technique____________________________________26
2.2.2 Generation, Amplification and Application of Femtosecond Light Pulses ______29
Experimental Setup_________________________________________________________34
3.1 Descriptions of Experimental Setup______________________________________34
Test Measurements _________________________________________________________47
4.1 Photoelectron Spectroscopy on Metals, Semiconductors, Adsorbates and Gases _47
Time-Resolved Photoelectron Spectroscopy on Pt(110) Surface _____________________54
5.1 Measurement of Hot Electrons on a Pt(110) Surface ________________________54
5.2 Pump-Probe Cross-Correlation in the EUV Region ________________________62
Time-Resolved Photoelectron Spectroscopy on GaAs(100) Surfaces__________________66
6.1 Measurements of Transient Ga-3d Core-Level Shifts on GaAs(100) Surfaces ___66
6.2 Dynamics of the band-bending for p-GaAs(100) ___________________________77
Contents
Conclusions and Outlook ___________________________________________________ 82
Appendix_________________________________________________________________ 84
A Knife-edge Beam Profile Measurement __________________________________ 84
B Auger Electron Spectroscopy on Pt, Cu and GaAs Surfaces _________________ 85
C Surface Photovoltage Transients and Surface Preparation __________________ 87
References _______________________________________________________________ 89
Acknowledgements
1
Acknowledgements
It was in summer 1998 when I was speaking for the first time with Prof. Ulrich
Heinzmann who got me an opportunity to join his working group and work on a project –
“Anwendung von mit Multilayern spektral selektierten hohen Harmonischen von fsLaserstrahlung für zeitauflösende Photoemissionexperimente” (Utilisation of high harmonics
selected with the multilayer optics for the femtosecond time-resolved photoemission
experiments). To him I owe an immense debt of gratitude for this great possibility to realise
this project. I thank him also for the possibility to be involved in a new SFB project “Physik
von Einzelmolekülprozessen und molekularer Erkennung in organischen Systemen” (Physics
of the single molecule processes and molecular recognition in organic systems). He gave me
unrestricted freedom in realising my experimental ideas. I really enjoyed having exercise
sessions for his lectures - Einführung in die Physik I (Introduction to Physics I), which were
really refreshing and enriching when compared with fade and repeating practical sessions.
At that summer I also met Dr. Markus Drescher who with excitement and full of
enthusiasm introduced to me his idea of the femtosecond visible-pump/extended ultravioletprobe (EUV) experiment. I wonder whether he knew at that time that this project is going to
be enormously successful. With his wide experiences I have realised pioneering experiments
on the field of femtosecond time-resolved photoelectron spectroscopy in the EUV region. We
have together achieved the first cross-correlation of femtosecond visible-EUV pulses on metal
surface (April, 2000) and the first time-resolved pump-probe core-level experiment on
semiconductor surface (July, 2001). With his great experimental skills and endurance he was
also main contributor to the fascinating cutting edge experiment in Vienna leading to the first
2
Acknowledgements
measurement of the single attosecond pulses. At this point I would like to say one great
“Thanks” for all what he has done for my work.
I am endless in dept to Prof. Štefan Luby and Dr. Eva Majková in Slovakia. They were
initiators of my PhD study in Bielefeld and they have continually supported my study and
experiments here in Bielefeld. I am looking forward to continuing this great and magnificent
cooperation.
It has been a great pleasure to work with former member of our group Dr. Michael
Spieweck. It seems almost incredible that we have built 0.2 TW femtosecond laser system in
only 18 months - a small miracle! He is also spiritual father of my German language skills. I
have never experienced better experimental cooperation than with him.
I gratefully acknowledge the work of our former diploma student Dip.-Phys. Tobias
Wiesenthal. He gave a birth to our “mechanical wonder” – monochromator for EUV
femtosecond pulses. He has done a good job!
Not less I thank Dr. YongCheol Lim who deposited multilayer mirrors for our
monochromator. These mirrors are until now world-wide unprecedented in their spectral
selection and low group velocity dispersion.
I would like to thank Dr. Norbert Müller for many helpful suggestions leading to the
better understanding of the experiments. His contributions to our experimental apparatus are
also immensely acknowledged.
I thank very much to Dipl.-Phys. Michael Sundermann and Dipl.-Phys. Armin
Brechling for their great collaboration on the self-assembled monolayers preparation.
My thanks to Dr. Frank Hammelmann and Prof. Peter Jutzi for providing me with
W(CO)6 and information on the subject of its vacuum deposition. Thanks to working group of
Prof. Jochen Mattay for synthesis of Br and I terminated self-assembled monolayers. Thanks
also to Prof. Markus Donath for supporting as with p-GaAs(100) crystal.
I wish to thank Dr. Ulf Kleineberg for his motivating and animating discussions. My
thanks go to former member Dr. Bernd Schmidtke who helped me occasionally with laser
alignment and was “real good brain” to tackle my small theoretical problems. I appreciate all
creative contributions from my close-working colleges Thorsten Uphues, Dipl.-Phys. Martin
Michelswirth.
My thanks go also to the members of our working group “Molekül- und
Oberflächenphysik”: Dipl.-Phys. Christian Meier for his technical help by “disobeying”
computers and his creative physical ideas, Dipl.-Phys. Martin Pohl for his critical remarks on
my technical drawings and introduction to the PEEM secrets, Kay Lofthouse for her
Acknowledgements
3
professional approach by the correcting of my english paper, Karla Schneider for her every
time helping hand in all kinds of bureaucratic problems, Volker Schimmang for his helping
by “mechanical” troubles, Dr. Oliver Wehmeyer for his web design, Dipl.-Phys. Thomas
Westerwalbesloh for his multilayer mirror calculations, Dipl.-Phys. Tarek Khalil for his
readiness to help, Dr. Wiebke Hachmann for her SEM images, Mag. Frank Persicke for his
introduction to the laser tweezers, Dipl.-Phys. Toralf Lischke for his Berlin’s “how to get
there?” knowledge, and Dipl.-Phys. Andreas Aschentrup for introduction to the LIP system.
Most importantly, my great thanks go to my entire family for support and
encouragement throughout my years at Bielefeld University.
4
Chapter 1 – Introduction
Chapter 1
Introduction
The first article1 in the column “Research Highlights” of the Nature Physics Portal
from 29th November, 2001 was published under the synonym - “The Age of Attophysics”. The
way to this scientific breakthrough2,3 is directly linked with the generation of light in the
extreme ultraviolet (EUV) region proposed some ten years ago4. Since then a rash progress5-7
has ended in the experimental evidence of a single attosecond pulse with a duration of
650±150 as. Considering that the attosecond pulse of such length is a coherent superposition
of electromagnetic radiation with a bandwidth§ of 2.8 eV or 680 THz, which is likely the
whole visible part of the electromagnetic spectrum, leads us to the conclusion that the carrier
of such ultra-short pulses would be the EUV region. Consequently, in the following years we
will be witnesses of the fast developing femto-and-attosecond time-resolved spectroscopy in
the EUV region.
The development8 of short laser pulses and their applications for time-resolved
measurements began in the early 70’s. The progress on laser pulse shortening is shown in
Fig. 1.1. The first lasers working in the femtosecond region have been dye lasers. Practically
all fascinating time-resolved measurements9-15 has been demonstrated in the 80’s. The fast
changes of the transmittance and reflectivity of semiconductors, metals and liquids linked
with inter- and intra-band transitions in semiconductors and metals, inter- and intra-molecular
transitions in complex organic molecules, the rearrangement and breaking of chemical bonds
§
For chirp-free gaussian pulse the relation between the bandwidth ∆ν and time duration ∆τ is given as
∆ν ⋅ ∆τ = 0.441
5
as well as the observation of phonons and excitons have been shown more than 10 years ago.
Although the dye laser stability has been lower when compared to contemporary femtosecond
lasers a pulse duration of less than 10 fs has been achieved with additional pulse spectral
broadening via a self-phase modulation and subsequent pulse compression16,17. In the early
90’s rapid development of femtosecond solid-state lasers resulted in the renaissance of the
“old” time-resolved techniques and stimulated growth of new and more sophisticated
measurements. Also the pulse duration18,19 of the femtosecond pulses in the visible has
reached its limits of some 4.5 fs.
Pulse duration
dye technology
Ti:sapphire technology
SPM & compression
high harmonics
Fig. 1.1 Road map of the laser pulse duration (taken from Ref. 20 and updated)
Parallel to these experiments in the visible, the nonlinear conversion of the visible light into
the EUV, soft X-rays and hard X-rays regions stimulated by the development of TW
femtosecond lasers21 has achieved great success and pushed the development of the theories
(Fig. 1.2). The classical nonlinear optics22 based on the perturbation theory being a powerful
tool of explaining χ (n ) processes has been not appliable to the new observed phenomena at
intensities higher than 1014 W/cm2. At these and higher intensities23 one is dealing with laser
electric fields comparable to those in the atoms themselves and the electrons become ionized
although the photon energies are much lower than the ionization potential of the atoms.
Effects such as above-threshold ionization, high harmonic generation but also high energetic
electrons24 and positrons25 generation has been observed and theoretically explained. Already
6
at currently available intensities of some 1020 W/cm2 one can observe relativistic effects in the
light-plasma interaction26-30.
Regimes of Nonlinear Optics
Perturbative regime
Strong-field regime
Bound electrons
Free electrons
χ Processes
(2)
Multiphoton
Second harmonic generation above-threshold
Optic parametric generation ionization
Optic rectification
χ(3) Processes
Laser ablation
Third harmonic generation
Stimulated Raman scattering
Long distance
Self-phase modulation
self-channeling
Self-focusing
1011
1012
1013
1014
1015
High harmonic
generation
Relativistic
regime
hard x-rays
Sub-fs x-ray and
electron pulses
Self-defocusing
1016
1017
Multi-MeV
electrons
Self-focusing
and channeling
1018
1019
Intensity (W/cm2)
Fig. 1.2 Overview of the new regimes of nonlinear optics (taken from Ref. 23)
Another challenge that is closely connected with the generation of the light pulses with
energies of few tens to few thousands of electronvolts is the measurement of their temporal
duration and possible application for ultra-fast time-resolved techniques. The chart showing
the progress on this field is compiled in Fig. 1.3. The pulse duration of the low-orders of high
harmonics with energies up to 13.5 eV has been successfully measured with autocorrelation
techniques based on two- and three-photon ionization31-33. However, this method is not
extendable to higher photon energies because of the extremely low transition probabilities of
the non-resonant two and more photon ionization by the current high harmonics intensities.
Recently, high harmonics up to 40 eV photon energies have been characterized with
cross-correlation techniques and utilised in time-resolved photoelectron spectroscopy
measurements on adsorbates34, isolators35 and in the gas phase36,37. Femtosecond pulse
duration of high harmonics with a photon energy of 70 eV has been measured via hot
electrons on a Pt surface in our Bielefelder-group described in chapter 5. Also stunning
measurement of attosecond pulse duration of the high harmonics with an energy of 90 eV has
been measured in the cooperation Vienna-Bielefeld1. The development of X-ray sources based
on plasma recombination generated with high power femtosecond laser pulses on solid state
7
surfaces initiated the first hard X-ray time-resolved diffraction measurements. Recently,
femtosecond pulse duration at photon energies of 4.5 keV38,39 and 8 keV40,41 were reported.
UV
1 eV
Soft X-rays
VUV
IR
Hard X-rays
Extreme Ultraviolet - EUV
10 eV
100 eV
Photon energy
1 keV
10 keV
Fig. 1.3 Chart of demonstrated femtosecond or sub-femtosecond pulse duration with
respect to their location in the electromagnetic spectrum.
The aim of this thesis is to present two measurements that are contributing to the rapid
growing field of femtosecond time-resolved spectroscopy in the EUV region. The first one
demonstrates the selection of a single high harmonic without temporal distortion in a
dedicated multilayer monochromator as proved by the measurement of its temporal duration
with a cross-correlation technique based on hot electrons at the Pt surface. I have already
shown a spectral selection35-47 of one high harmonic order. In chapter 5 I will report on
the measurement48-50 of the 45th high harmonic temporal duration. The peak photon flux of
built high harmonic source compared to that of 3rd generation storage ring51 (BESSY II) is
shown in Fig. 1.4. The main advantage of high harmonic source as compared to such large
scale facilities is the femtosecond time duration of high harmonic pulses. There are already
first advances52,53 of a formation of synchrotron radiation with femtosecond pulse duration but
until now no experiment has been demonstrated utilizing these pulses. The current drawback
of the high harmonic sources is the low repetition rate of a few kHz compared to that of few
hundred MHz of the storage rings51. The second significant measurement presented in
chapter 6 is the first application of a spectrally selected single high harmonic order for
time-resolved core-level photoelectron spectroscopy. Recently published papers34-36 on
time-resolved photoelectron spectroscopy using high harmonics are dealing exclusively with
the valence bands or highest occupied molecular orbitals. Here I report on the first
application54-57 for a femtosecond time-resolved study involving core-level electrons on the
GaAs surface.
8
The extreme surface sensitivity of the photoemitted Ga-3d core-level electrons served for an
observation of the charge transport and recombination after photoexcitation with the visible
pump pulse.
Peak flux
High Harmonic
e
Fig. 1.4 Performance of built high harmonic source compared to BESSY II (based on data
from Ref. 51)
Chapter 2 – Theoretical Background
9
Chapter 2
Theoretical Background
2.1 Light–Solid Interaction
2.1.1 Photoexcitation and Photoemission
Under photoexcitation one understands the process when an electron after absorption
of a photon will be excited into a binding state localized in the bulk or on the solid state
surface. Photoemission stands for the process where the electron is excited into the continuum
and escapes to vacuum. A simplified model of the metal and semiconductor electronic
structure§ is shown in Fig. 2.1. The lowest photon energy needed for the photoemission is
defined as the work function φ for the metals and χ + E g for the semiconductors, where χ is
the electron affinity and E g is the semiconductor band gap.
The probability of the photon absorption and electron transition from the initial state i to the
final state f is given by60
Pfi =
§
2π
h
2
f H i i δ ( E f − Ei − hω )
(2.1)
The negative binding energies throughout this thesis are given relative to the Fermi level for the metals; relative
to the top of the valence bands for semiconductors; and relative to the vacuum level for the rare gases.
10
Metal
Semiconductor
Ebin(eV)
Ebin(eV)
Evac
Evac
CBM
φ
EF
EVBM=0 VBM
EF=0
core-levels
χ
Eg
core-levels
Fig. 2.1 Scheme of the electronic structure of metal and semiconductor
Ei and E f are the energies of the initial and the final electron states respectively and
H i = e m k ⋅ A , where k is the electron momentum operator and A is the vector potential.
The energy conservation law is enforced with the delta function. In the case of the
photoemission when the final state is not a bound state, the kinetic energy of the free electron
is given by (2.2) and (2.3) for metals and semiconductors respectively
E kin = hω + Ei − φ
(2.2)
E kin = hω + Ei − χ − E g
(2.3)
However, the probability of the photoexcitation and photoemission in the solid state is not
given with the simple equation (2.1), but has to include the specific electronic structure of the
examined specimen. After correction due to the variety of initial and final states and their
dispersion with the electron momentum one can define the imaginary part of the dielectric
function60, which is proportional to the absorption coefficient62 by the photoexcitation and to
the photoemission cross section in the case of photoemission63,64 respectively, as follows
1 e 
Im(ε (ω )) = 

ε 0  mωπ 
2
∑∫
i, f
2
f (k ) H i i (k ) δ ( E f (k ) − Ei (k ) − hω )d 3k
(2.4)
photoexcitation
11
photoemission
Fig. 2.2 Imaginary part of the dielectric function for Pt and GaAs (based on data from Ref. 62)
Fig. 2.3 Electron inelastic mean free path for Al and GaAs (taken from Ref. 67, 68) and their
lattice constants (taken from Ref. 61)
12
For illustration the imaginary parts of the dielectric function62 for two relevant solid states, Pt
and GaAs, are shown in Fig. 2.2. Only the photons with an energy exceeding approx. 5.5 eV
cause photoemission from the Pt and GaAs surfaces. Described as a three-step process65, the
photoemission consists of excitation, transport and escape of electrons from the solid state.
During transport to the surface the final kinetic energy is reduced due to electron-electron
(e-e) scattering and electron-phonon (e-p) scattering. As consequence of the inelastic
scattering processes the collision mean free path of the electrons is drastically reduced as seen
in Fig. 2.3. Electrons with a kinetic energy of about 50 eV typically possess an inelastic mean
free path of less than 1 nm66-68. Correspondingly, techniques probing electrons with kinetic
energies around 50 eV are particularly surface sensitive, because 86% of all electrons are
originating from the first and second atomic layer of the solid state. A characteristic feature of
the low kinetic energy electron spectra is a huge secondary electron background as result of
the inelastic scattering processes.
E kin = mhω − φ
Ebin
MPE
Evac
hω
φ
EF
Fig. 2.4 Multiphoton photoemission processes
As already noted, the photons with energies less than the work function for the metals
or sum of the electron affinity and the band gap energy for the semiconductors can not
promote electrons into vacuum. The situation changes when a huge amount of photons in a
short time participate in the photoexcitation processes and nonlinear processes can be
observed. Typical many-photon processes are summarized in Fig. 2.4. Let us speak about
multiphoton emission69 (MPE) when an absorption of an appropriate number of photons
stimulates the electron transition over the vacuum barrier. At even higher light intensities an
electron that is already free due to MPE undertakes additional photon stimulated transitions in
13
continuum resulting in its final kinetic energy is mhω − φ , where m is the number of the
absorbed photons70 (see Fig. 2.4).
2.1.2 Relaxation Mechanisms in Metals
In general the relaxation mechanism describes the transition of the excited system back to
the equilibrium. Such mechanisms are usually characterized with the energy and the phase
relaxation times71-73.
1
hω10
0
Fig. 2.5 Scheme of a two level system coupled to the heat sink
Suppose we have brought the simple two-niveau system (Fig. 2.5) in the excited state with the
resonant light pulse. The relaxation induced via a dissipative system (heat sink) is described
by the following equations in the density matrix formalism73
d
1
ρ10 (t ) = −iω10 ρ10 (t ) − ρ10 (t )
dt
T2
(2.5)
d
1
ρ11 (t ) = − ( ρ11 (t ) − ρ11( e ) )
dt
T1
(2.6)
ρ 01 (t ) = ρ10* (t )
(2.7)
ρ 00 (t ) = 1 − ρ11 (t )
(2.8)
where ρ11( e ) denotes the thermal equilibrium and T1 , T2 are the energy and the phase
relaxation times, respectively. The elements of the density matrix ρ ij can be calculated as
14
ρ ij = i ρˆ j
i, j = 0,1
(2.9)
where ρ̂ is the density operator. The energy relaxation time T1 characterizes the occupation
decay ρ11 (t ) of the upper level of the two-niveau system
ρ11 (t ) ~ exp(−
t
)
T1
(2.10)
The phase relaxation time T2 describes the decay of the induced polarization P in the system
with the excitation light pulse
P ~ p10 ρ 01 + p01 ρ10 ~ exp(−
t
)
T2
(2.11)
where p10 and p01 are the matrix elements of the atomic dipole operator. The energy
relaxation time as well as the phase relaxation time are experimentally observable.
0,1
1
10
100
1000
One optical cycle of
400 nm light (1.33 fs)
Dynamical
screening
Electronic decoherence
e-e Scattering
Surface state quenching
e-p Scattering
0,1
1
10
Time scale (fs)
100
1000
Fig. 2.6 Chart of the time response of metals to light excitation (taken from Ref. 72)
Most fundamental processes governing the excitation and relaxation processes in metals are
summarized in Fig. 2.6. Dynamical screening describes the primary response of the valence
electrons to the excitation light pulse. A macroscopic effect manifesting the dynamical
15
screening of electrons is the polarization of metals. Therefore all fundamental effects such as
light reflection but also non-linear effects like second harmonic generation depending on the
polarization of metals evolve on the attosecond time scale72,74. Loss of the collective
excitation coherence in the absence of the driving excitation light pulse is referred to as
electronic decoherence72. The phase relaxation time T2 in the equations (2.5)-(2.8) is
describing the loss of coherence in the observed system. Macroscopically, it can be observed
as an exponential decay of the light-induced polarization of metals. In the last decade the
interferometric time-resolved two-photon photoemission technique made direct observation of
the electronic decoherence possible76.
The reason for the electronic decoherence are mutual interactions of electrons such as
the impurity and e-e scattering72,77. In the case of electron scattering by charge impurity the
electrons interact with charge impurities through the Coulomb potential77. This process is
elastic, that is the electron energy is conserved and only the direction of the electron
momentum is changed. The electrons with lower kinetic energy are deflected more than
electrons with higher kinetic energy by charged impurities. One of the most effective
processes leading to decoherence is the e-e scattering72. To clarify the e-e scattering let us use
the concept of hot electrons, which are electrons excited above the Fermi level. A hot electron
with momentum k1 interacts with one electron below the Fermi level with the momentum k 2
through the Coulomb interaction to produce two hot electrons with energies less than the
energy of the primary hot electron and momentum k1′ and k 2′ . The electron energies and
momenta are given by
k1 + k 2 = k1′ + k 2′
(2.12)
E (k1 ) + E (k 2 ) = E (k1′) + E (k 2′ )
(2.13)
The characteristic e-e scattering time τ e−e for hot electrons with the initial energy difference
to the Fermi level E − E F derived by Fermi liquid theory (FLT) is72
τ e −e
 EF
= τ 0 
 E − EF



2
(2.14)
where τ 0 is given exclusively by the density of the electron gas72. Direct measurement of the
e-e scattering times / hot electron life times has been performed with the time-resolved two-
16
photon photoemission technique for series of noble and transition metals78-89 but also with
traditional optical linear and non-linear time-resolved techniques94-97. Typical measured hot
electrons relaxation times89 for noble metals such as Ag and transition metals as Ni are shown
in Fig. 2.7.
Fig. 2.7 Measured hot electron relaxation times for Ag and Ni metals (taken from Ref. 89)
A very important electron energy loss mechanism in the femtosecond and picosecond
region is the e-p scattering71,72,77. Populating the phonon modes in the e-p scattering leads to a
rise of the lattice temperature. Typical energies of the phonons are a few tens of meV and
therefore a lot of electron-phonon interactions are needed to lower the hot electron energy
considerably. Phonons are acting as a heat sink for the thermalized hot electrons. The
dynamics of the electrons and phonons is described by the following coupled differential
equations in terms of the electron gas temperature Te
and phonon gas (lattice)
temperatures Tl 98-100
Ce
∂
Te = ∇ r (κ ⋅ ∇ r Te ) − g (Te − Tl ) + G (t )
∂t
(2.15)
Cl
∂
Tl = g (Te − Tl )
∂t
(2.16)
17
where Ce and Cl are the heat capacities of the electrons and lattice, G (t ) is the optical
generation term and g is the electron-phonon coupling constant. The hot electron diffusion
term with the electronic heat conductivity κ is dominant in the first few picoseconds and
therefore the term describing phonon diffusion has been neglected. To illustrate the “cooling”
of hot electrons after initial pulsed photoexcitation of the platinum surface with an intensity of
32 GW/cm2 the electron and lattice (phonon) temperatures are shown in Fig. 2.8. Rapid
cooling of the thermalized hot electrons can be observed in the first few picoseconds.
I = 32 GW/cm2
Fig. 2.8 Cooling of the electron gas via e-p scattering after photoexcitation with laser pulse on
the Pt surface (taken from Ref. 100).
All time-resolved techniques probing surface dynamics are measuring electron life
times on surfaces, therefore surface-states85,86 which are real states localized on the surface
are playing a significant role in the hot electron relaxation. There is possibility of transfer of
hot electrons to the surface-states and so to prolong electron life times. Also an electron
transfer to empty states of adsorbates has been observed and measured72,85,91-92. Auger
processes leading to energy relaxation of hot-electrons in metals have been recognized as
well72.
A very important aspect of the electron relaxation in metals are transport effects101,103.
Almost every technique used to measure hot electron life times is surface sensitive, therefore
electron transport into the bulk is affecting measured relaxation times104. Summarized
18
electron relaxation processes together with the electron diffusion into the bulk are shown in
Fig. 2.9. Pulsed photoexcitation promotes electrons into the unoccupied states above the
Fermi level and produces a hot-electron population. At the same time the electrons are
ejecting into the bulk and leaving the surface region with the velocity ~ 1 nm/fs (Fig. 2.9a).
laser
a)
surface
DOS
electron
density
hν
t=0
v ~ 1 nm/fs
EF
E
z
nonequilibrium electrons
b)
electron
density
DOS
kTe
t = τe-e
Te > T l
EF
electrons in thermal
equilibrium
c)
ballistic electron
motion
v ~ 10 nm/ps
E
z
hot electron
diffusion
electron
density
DOS
t = τe-p
Te = T l
kTl
v ~ 100 µm/µs
EF
E
z
thermal diffusion
electrons and lattice
in thermal equilibrium
Fig. 2.9 Relaxation and transport of hot-electrons in metals (taken from Ref. 103)
In a short phase of few tens of femtoseconds after photoexcitation the hot electrons thermalize
via e-e scattering and are characterized with the electron temperature Te . During this time the
lattice temperate Tl is much smaller than the electron temperature Te , while the surface
region is depleting of electrons with the velocity of 10 nm/ps (Fig. 2.9b). The third phase of
the electron relaxation is characterized through the e-p scattering when “lattice heating”
occurs due to thermalization of the electrons and phonons (Te → Tl ). Typical electron
transport velocities are about 100 µm/µs which is conventional thermal diffusion.
19
2.1.3 Relaxation Mechanisms in Semiconductors
There are many similarities of relaxation processes in semiconductors with the
processes in metals. The main differences have their origin in the band structure of the
semiconductors and the correspondingly smaller density of free electrons. An overview of
typical relaxation processes and their characteristic times is given in Fig. 2.10.
1 fs
10 fs
100 fs
1 ps
10 ps
1 ns
100 ps
10 ns
100 ns
1 µs
carrier-carrier scattering (1)
intervalley scattering (3)
intravalley scattering (2)
radiative
recombination (5)
Auger recombination (4)
carrier diffusion
defect recombination (6)
1 fs
10 fs
100 fs
1 ps
10 ps
initial electron
population distribution
1
1 ns
100 ps
10 ns
100 ns
1 µs
3
4
2
CBM
4
hν 5
6
defect state
VBM
Fig. 2.10 Electron relaxation processes in semiconductors (taken from Ref. 105,106)
The coherent regime initiated with the photoexciting pulse is similar to the one in metals and
is described with the coherence dephasing time (see eq. (2.11)).
(arb. un.)
20
Fig. 2.11 Time-resolved change of the GaAs transmittance for 1017 cm-3 photoexcited
carrier densities (adopted from Ref. 107)
Rapid loss of coherence has the same reason as in metals and is primarily connected with the
fast e-e scattering71. The e-e scattering is broadening the initial electron population
distribution to the hot thermalized distribution (Fig. 2.10-1). At this stage the electron gas
temperature is different from the phonon gas (lattice) temperature. The following phase is
characterized through the e-p scattering108. Phonon scattering processes are decreasing the
electron gas temperature and increasing the lattice temperature. In semiconductors the e-p
scattering is divided into two main groups – intervalley scattering and intravalley scattering.
In intravalley scattering the electron - after interacting with phonon - stays in the same
valence band valley (Fig. 2.10-2). The intervalley scattering transfers the electron to a
different valley (Fig. 2.10-3). So electrons are scattering to the bottom of the conduction band
where much slower processes occur, such as radiative recombination (Fig. 2.10-5), Auger
recombination (Fig. 2.10-4), defect recombination (Fig. 2.10-6) in the bulk or the
recombination via surface states. To illustrate the discussed processes Fig. 2.11 shows the
time-resolved differential transmission measurement of a 0.5 µm GaAs film for 1017 cm-3
excited carrier density107. A fast decrease of the transmittance in the first few femtoseconds is
attributed to e-e scattering whereas a slower decay is pointing to the e-p scattering.
21
Time-resolved studies based on two-photon photoemission has been applied to measure the
electron dynamics on semiconductors surfaces as well109-111.
The process of carrier diffusion is much more complex for semiconductors as
compared to metals in the surface vicinity. The charge localized in the surface states (SS) is
compensated via bulk electrons. For metals with a high density of electrons in the valence
band this compensation is resulting in a dipole layer of only a few angstroms near the
surface131. Semiconductors with lower electron density in the valence band are building a
space-charge region (SCR) which can extend few tens of nanometers into the bulk. Within the
SCR the surface charge is compensated, leading to a band-bending in the surface
vicinity132-138. To shed more light on the phenomenon of band-bending let us suppose a n-type
semiconductor with acceptor-type surface states. Electrons will be transferred to the empty
surface states which results in a negative surface charge QSS . To achieve equilibrium we need
the same amount of positive space charge QSC under the surface to satisfy the net charge
neutrality136
QSC + QSS = 0
(2.17)
As a result, the depleted zone of electrons – SCR will be formed. All potentials in the SCR
have to fulfill Poisson’s equation136 for the new charge redistribution which results in an
upward band-bending towards smaller effective binding energy. For p-type semiconductors
with the donor type of surface states the situation will reverse whereby the surface charge QSS
is positive and the space charge QSC is negative. In this case all bands bend downwards. In a
first approximation the SCR width w , the doping density N and the band-bending Y0 at the
surface are coupled as follows136,139
w=
2εY0
eN
(2.18)
where ε is the dielectric constant and e is the elementary charge. For example the p-GaAs
with N =1019 cm-3 and Y0 =0.7 V has the SCR length of 10 nm. The potential change of 0.7 V
over the range of 10 nm is responsible for very high electric fields of few hundreds of kV/cm
in the SCR139. Additional free electrons and holes injected by light absorption can change the
band-bending. This phenomenon is called surface photovoltage (SPV) effect132-138.
22
Ebin
t
e
S
-
-
-
-
-
-
Auger
surface
recomb.
Y
Y0
Vacuum
+
+
+
defect
(SRH)
radiative
E
SS
EF
+
+
+
CBM
VBM
+
+
p-GaAs
SCR
z
Fig. 2.12 Scheme of the band-bending for p-GaAs
The dynamics of the photogenerated carriers - electrons and holes - neglecting the
mentioned relaxation processes such as e-e and e-p scattering are described by the following
coupled differential equations139-143
∂n( z , t )
∂ 2 n( z , t )
∂
= G ( z , t ) + Dn
+ µ n [E ( z , t )n( z , t )] − Rn
2
∂t
∂t
∂z
(2.19)
∂p ( z , t )
∂ 2 p( z, t )
∂
= G ( z, t ) + D p
− µ p [E ( z , t ) p ( z , t )] − R p
2
∂t
∂t
∂z
(2.20)
∂ 2V ( z , t ) ∂E ( z , t ) e
=
= [ p ( z , t ) − n( z , t ) + N d ( z ) − N a ( z ) ]
∂z 2
∂z
ε
(2.21)
The time-resolved band-bending Y = V ( z = 0, t ) is given by the time-depended densities of
electrons n( z , t ) and holes p (z , t ) as well as by the densities of ionized donors N d and
acceptors N a . The dynamics of the electron density n( z , t ) and hole density p ( z , t ) determine
the continuity equation where G ( z , t ) is the optical generation term, Dn and D p are the
diffusion coefficients for electrons and holes, µ n and µ p are the electron and hole mobility.
Recombination terms Rn and R p are describing recombination processes143 such as radiative
recombination139, defect – Shockley-Read-Hall recombination144 and Auger recombination143.
23
Surface recombination dynamics is given by two boundary conditions for electrons and
holes143
∂n
∂z
∂p
∂z
=
z =0
=
z =0
( S n + S nt − µ n E ( z = 0))
n( z = 0, t )
De
( S p + S tp − µ p E ( z = 0))
Dp
p ( z = 0, t )
(1.22)
(1.23)
where S n and S p are the surface recombination velocities and S nt and S tp are the carrier
transfer velocities characterizing processes like tunneling or thermionic emission. Equations
(2.19)-(2.23) have no analytic solution143. To illustrate the competing processes evolving after
photoexcitation of a semiconductor surface let us suppose a p-type GaAs semiconductor
surface (Fig. 2.12). Pulsed photoexcitation with a photon energy larger than the band gap
produces electron-hole pairs. The presence of the high surface electric field separates the
photogenerated electrons and holes, whereby the electrons move to the surface and the holes
move further into the bulk. The change of the carrier density in the SCR flattens the original
band-bending Y0 to a new value Y . The electron mobility saturates139,140 for electric fields of
some 100 kV/cm, for even higher fields the drift velocity therefore does not exceed 50 nm/ps.
In this simple estimation the electron needs about 200 fs for transit through the SCR of 10 nm
width145.
Fig. 2.13 Spatio-temporal evolution of the space charge field (taken from Ref. 169)
24
Fig. 2.14 Normalized time-resolved THz pulse wave and its spectrum (taken from Ref. 167)
The result of a detailed numerical study169 of the space charge field after delta-like
pulsed photoexcitation of n-GaAs for the low carrier excitation density of 4x1016 cm-3 and the
low doping concentration of 1015 cm-3 is shown in Fig. 2.13. The decrease in the space charge
field accompanied with an oscillatory behavior is pointing to a complex band-bending time
evolution already at relatively small excitation intensities. Fast electron transfer in the SCR
immediately after photoexcitation leads to radiative emission of the broad-band femtosecond
far-infrared (FIR) THz pulses170-179. The FIR field strength due to transient charge transfer is
given directly as the second derivative of the space charge field167. Measured FIR-THz pulse
at different excitation densities of 1x1015, 1x1016 and 1x1017 cm-3 in p-GaAs (4x1014 cm-3)
together with their spectra are shown in Fig. 2.14. Femtosecond optically generated THz
pulses in the SCR of semiconductors have found widespread application in THz time-resolved
studies180-183.
Once the electrons and holes have been separated and a new value of the band-bending
has been achieved the relaxation processes take place. Besides the bulk recombination
mechanisms like radiative, defect or Auger recombination, the surface offers new
recombination possibilities like the surface recombination via surface states. The surface
states act as recombination centers in the same manner as defect states144 in the bulk. Surface
recombination146 can be relatively fast on the picosecond time scale147-153. Direct
measurements of the SPV dynamics on nanosecond and coarse picosecond time scales have
already been published154-164. These were based on an excitation laser synchronized with the
probing soft x-ray storage ring pulses. In spite of these advances the fast SPV transients on
the p-GaAs(100) surface have not been resolved as shown in Fig. 2.15. The SPV effect of the
25
anisotropic materials such as GaAs has been also measured with the pure optical
time-resolved methods142,166-168 making use of the electrooptic effect165 on the surface.
Fig. 2.15 SPV shift of the Ga-3d photoelectrons in p-GaAs at 125 K as a function of the delay
time between soft x-ray synchrotron pulse and excitation laser pulse (taken from Ref. 161)
26
2.2 Pump-Probe Technique
2.2.1 Principle of Pump-Probe Technique
One
can
73,184
techniques
observe
ultrafast
dynamics
applying
the
so-called
pump-probe
. The general principle common to all types of pump-probe techniques is
schematically shown in Fig. 2.16.
time
τ - delay
probe
pulse
pump
pulse
system in
equilibrium
excitation
of the system
relaxation
detection of
photons or
electron
Fig. 2.16 General scheme of pump-probe techniques
The system initially in the equilibrium is optically excited into unoccupied states with the
pump pulse. Once excited, the system relaxes back to the equilibrium states with the
characteristic relaxation times (see section 2.1.2). A probe pulse with variable time-delay with
respect to the pump pulse “maps” the changes in the relaxing system. Photons or electrons
gated (=photoemitted, photodiffracted, up-converted etc.) by the probe pulse reflect the actual
state of the system at the time of the probe pulse arrival. Scanning the time delay τ between
pump and probe pulses one can learn about the relaxation dynamics of the system.
Let us assume that relaxation of the system back to the equilibrium is described by the
function f (t ) after the excitation with the delta-like pump pulse δ (t ) . The actual response of
the system g (t ) is described by the following convolution73,165 with the pump pulse’s
temporal intensity profile h pu (t )
27
+∞
g (t ) = f (t ) × h pu (t ) =
∫ f (ξ ) ⋅ h
pu
(t − ξ )dξ
(2.24)
−∞
or Fourier transformed
G (ω ) = F (ω ) H pu (ω )
(2.25)
The measured signal m(t ) gated by the time delayed probe pulse with the temporal intensity
profile h pr (t ) can be written as convolution in the form
+∞
m(t ) = g (t ) × h pr (t ) =
∫ g (ξ ) ⋅ h
pr
(t − ξ )dξ
(2.26)
−∞
or Fourier-transformed
M (ω ) = F (ω ) H pu (ω ) H pr (ω )
(2.27)
Now we can distinguish two limiting cases which simplify the derived equation (2.27). Let us
assume that pump and probe intensity pulse profiles are delta-like functions. With this
simplification the measured signal m(t ) equals the function f (t ) . It means that we measure
exclusively the relaxation of the investigated system without any influence of pumping and
probing pulses. Another limiting case is when the relaxation function f (t ) is a delta-like
function. The measured signal m(t ) now equals a cross-correlation of pump and probe pulses.
This case gives an opportunity to measure the linear cross-correlation signal when the pump
duration is considerably larger than the relaxation time of the system.
In pump-probe techniques one measures the photons or the electrons directly gated
with the time delayed probe pulse under the assumption that the intensity of the probe pulse is
smaller than that of the pump pulse. This means, that the influence of the probe pulse on the
relaxation dynamics will be negligible. In practice, the following observables can be detected:
-
absorption, transmission, reflection, diffraction, polarization of the “probe” photons
-
nonlinear effects like second harmonic generation (SHG) of the “probe” photons
-
fluorescence photons gated with the “probe” photons
-
photocurrent due to photoexcited electrons by the “probe” photons
28
-
photoelectrons emitted by the “probe” photons
Another difference between various pump-probe techniques is the propagation of pump and
probe pulses. There are two principal possibilities of the mutual pump and probe propagation
- collinear and non-collinear. Finally, time-resolved techniques can be phase sensitive or
intensity sensitive. The phase sensitive time-resolved techniques are pushing the
time-resolution under the duration of the pulse optical period cycle (400 nm = 1.33 fs)76.
A special class of the time-resolved techniques is the time-resolved photoelectron
spectroscopy (TR-PES). This technique combines the classical photoelectron spectroscopy
with the possibility to directly observe the transient changes of the electronic structure after
photoexcitation. Photoexcitation of the valence electrons results in a transient population of
normally unoccupied orbitals of molecules or bands of the solid state. The TR-PES techniques
specializing on these transient electron populations utilize probe photons in the visible or
ultraviolet region. A typical representative of this TR-PES class is the time-resolved twophoton photoemission (2PPE) technique72. Its principle is based on the following scheme for
metals or semiconductors. The pump pulse excites the valence electrons into the unoccupied
states above the Fermi level. After photoexcitation the probe pulse causes photoemission from
the transiently occupied states. This technique provides one immediately with the relaxation
times of the transiently occupied states. It has been developed for metals78-89,
semiconductors109, clusters112 and molecules113. Exciting electrons into the unoccupied states
can result in the photodissociation or photoisomerization of molecules, charge transport on
the surfaces in semiconductors, desorption of adsorbates on metal surfaces and so forth. All
mentioned processes can be detected as chemical shifts304 in the core-levels or changes in the
valence band structures. Consequently, the probe photon energy must be in the vacuum
ultraviolet (VUV) or EUV region for the valence band and core-level studies, respectively.
The latter condition requires femtosecond pulses at least in the EUV region. Pioneering work
in the field of EUV TR-PES was performed by Haight et al. who studied photoexcited
semiconductor surfaces with few hundreds of femtoseconds resolution114-130. Recently, this
techniques has also been applied to the adsorbate dynamics on a Pt surface34, the molecular
dissociation of Br236,37, and hot-electron dynamics in quartz35.
29
2.2.2 Generation, Amplification and Application of Femtosecond Light
Pulses
A light pulse of the femtosecond duration is a superposition of large number of the
planar waves with different wavelengths but well defined phase conditions. Such pulse can be
conveniently described in the following form73
E (t ) =
1
2π
+∞
∫ E (ω )e
i φ (ω )
⋅ e iωt dω
(2.28)
−∞
where E (ω ) is the amplitude and φ (ω ) is the phase of the complex spectral density
E(ω) = E (ω )e iφ (ω ) . Generally the phase φ (ω ) can be written as185
1
1
φ (ω ) = φ (ω 0 ) + φ ′(ω 0 ) ⋅ (ω − ω 0 ) + φ ′′(ω 0 ) ⋅ (ω − ω 0 ) 2 + φ ′′′(ω 0 ) ⋅ (ω − ω 0 ) 3 + ...
2
6
(2.29)
where φ ′′(ω 0 ) is called group velocity dispersion (GVD) term, φ ′′′(ω 0 ) is called third order
dispersion (TOD) term and so on. A pulse with a linear phase φ (ω ) is a so-called
Fourier-limited pulse and is the shortest achievable pulse for a given bandwidth E (ω ) . The
action of a linear optical system on femtosecond light pulses is given by the convolution of
the impulse response function s (t ) of the linear optical system73,75,165 and the femtosecond
pulse E (t ) and after Fourier transformation is simply given in the form
Eout(ω) = S (ω )e iϕ (ω ) Ein(ω)
(2.30)
where Ein(ω) is the complex spectral density of the in-going pulses and Eout(ω) is the complex
spectral density of the transformed pulses leaving the system. The linear system changes the
amplitude of the complex spectral density by the S(ω) and introduces the phase-shift ϕ(ω).
Let us suppose that the linear system acts solely in the phase domain. In this case S (ω ) = 1 .
As result of such transformation the Fourier-limited pulse after the propagation through the
linear system receives GVD ≡ ϕ ′′(ω 0 ) , TOD ≡ ϕ ′′′(ω 0 ) and higher orders of the linear system
phase components. In other words, the pulse will stretch in time via accumulated phase-shifts
30
while propagating through the linear system. This is illustrated in Fig. 2.17 for the linear
system with S (ω ) = 1 .
∆ωin
ω
∆ωout=∆ωin
linear system
ω
Eout=S(ω)eiϕ(ω)Ein
∆τin
t
∆τout≠∆τin
t
Fig. 2.17 Femtosecond pulse transformation through the linear system
A Fourier-limited pulse stretched by the linear system with a positive (negative) GVD term is
called up-chirped (down-chirped)73. The GVD term is responsible for a symmetrical
broadening of the pulses whereas the TOD term causes an asymmetrical broadening.
Contributions of higher terms than TOD are important only for few-cycles optical
pulses21,23,186.
Generation of femtosecond pulses requires a laser medium with a broadband spectral
gain like the Ti-doped sapphire crystal. This medium exhibits a sufficiently large emission
spectrum to support pulses even less then 10 fs duration20.The principle of femtosecond pulse
generation is based on the locking of the phase relationship between different laser modes in a
cavity which is random under normal circumstances. To achieve a fixed phase relationship
one can use active or passive mode locking techniques73. In most cases the passive Kerr-lens
mode locking (KLM) technique23,187-190 is used to generate the femtosecond light pulses. An
artificially induced perturbation of the laser cavity leads to sudden high intensity fluctuations.
Their propagation in the specially designed laser cavity is more favourable due to the Kerr
effect and the laser starts pulsed operation. Further amplification of the femtosecond laser
pulses without any additive arrangements would lead to a destruction of the laser media and
optics due to the very high intensity leading to self-focusing and other nonlinear effects. The
chirped pulse amplification14,191-207 (CPA) technique tackles this problem of material damage
by stretching – “up-chirping – of the femtosecond pulses before amplification and
recompressing – “down-chirping” – of the amplified pulses back to the initial short pulse
duration. In detail, the femtosecond pulses from the master oscillator are stretched in an
optical device with very high positive GVD – the stretcher. Such long pulses are routinely
31
amplified in two types of optical amplifiers – regenerative and multipass amplifiers.
Regenerative amplifiers consist of an optical cavity with an optical relay being able to confine
and release optical pulses within the cavity. In this way one can regulate the number of passes
through the amplifying medium. A disadvantage of the regenerative amplifiers is the
relatively large GVD and higher dispersion terms leading to an additional hard controllable
stretching of the pulses208. On the other hand, the multipass amplifiers have very low
dispersion but are not flexible in the choice of the number of passes through the amplifying
medium which are realized as “optical-bench-fixed” paths multiplexing at small mutual
angles in the amplifying medium. After amplification the pulses are recompressed back to
almost the original duration by a device with large negative GVD – the compressor.
The high intensities of femtosecond light pulses are predestining them for the
stimulation of nonlinear optical effects22,58,59. Classical nonlinear optics is based on the
perturbation theory22. At low intensities (< 1013 W/cm2) nonlinear phenonena23 have been
successfully described with the Maxwell equations and a medium polarisation ansatz73
P = ε 0 χ ( E ) E = ε 0 χ (1) E + ε 0 χ ( 2) E 2 + ε 0 χ ( 3) E 3 + ... + ε 0 χ ( n ) E n
(2.31)
where χ (n ) are known as the nonlinear optical susceptibilities of nth order. A representative
class of the second order nonlinear effects like second harmonic generation, sum frequency
mixing, optical rectification, optical parametrical generation are governed by the χ ( 2 )
susceptibility. Third harmonic generation, self-focusing, two-photon absorption, self-phase
modulation are modelled on the χ (3) susceptibility. At higher light intensities (> 1013 W/cm2)
the light electric field is not merely a small perturbation of the atomic Coulomb potential. To
describe the nonlinear effects accompanying such high light intensities one has to perform an
ab initio quantum mechanical calculation of the system atom and high intensity light field.
The key-process powering all experiments throughout this thesis is the high harmonic
generation, theoretically209-224 and experimentally225-262 explored over one decade now. In this
process the high harmonic photons in the VUV, EUV and soft X-ray region are produced by
the interaction of the intense laser field (>1014 W/cm2) with an atomic gas. The classical
interpretation of the high harmonic generation is based on a three-step model209: In the first
step, an electron tunnels through the Coulomb barrier suppressed by the intense laser field.
Once free, the electron is oscillating and gaining energy from the laser field and under certain
32
circumstances recombines with the mother ion. The electron recombination process209 attends
the photon emission with the energies
Ehh = n ⋅ hω
(2.32)
where ω = 2π / T is the laser field frequency and n is odd integer number. To estimate the
maximum extractable photon energy we can use the classical trajectory209-211 x(t , t 0 ) of the
free electron born at the time t 0 in the laser field E = E sin(ωt )
x(t , t 0 ) = x0 +
v0
ω
(sin(ωt 0 ) − sin(ωt )) + (t − t 0 )v0 cos(ωt 0 )
(2.33)
The kinetic energy of the oscillating electron in the laser field is then given simply as
[
E kin (t , t 0 ) = 2U P cos 2 (ωt ) − 2 cos(ωt ) cos(ωt 0 ) + cos 2 (ωt 0 )
]
(2.34)
Electron kinetic energy
in UP units
no recombination
possible
no recombination
possible
Electron Recombination
Path
Fig. 2.18 Kinetic energy (contour plot) and recombination times (shown as white paths) of the
free electron born at the time t0 in the laser field with period T.
33
where v 0 = eE mω and U p = e 2 E 2 4mω 2 is the ponderomotive energy of the electron in the
laser field. To estimate the maximum energy of the emitted high harmonic photons we have to
know the actual kinetic energy of the electron colliding with the mother ion. The contour plot
in Fig. 2.18 shows the electron’s kinetic energy E kin (t , t 0 ) given by equation (2.34) in the
oscillating laser field at arbitrary normalized time t / T born in the laser field E at the
normalized time t 0 / T . Solutions of the equation (2.33) for x(t , t 0 ) = 0 shown in Fig. 2.18 as
white “recombination paths” constitute the only possible recombination times for electron
with mother ion in the oscillating laser field. The graphical solution (Fig. 2.18) of the electron
recombination process makes clear that only electrons born in the second and fourth quarter
of the laser field period will return to the mother ion and can radiative recombine. The
electrons born in the first and third quarter of the laser field period will never return to the
mother ion again and therefore do not contribute to the high harmonic generation (shown by
hatched area in Fig. 2.18). The electrons born in the laser field at the times t 0 = {0.3T ,0.8T }
will return to the mother ion 0.64T period later with the maximum possible kinetic energy
3.17U P obtained in the laser field (shown with the arrow in Fig. 2.18). Radiative
recombination for this case constitutes the cutoff energy for the high harmonic photons
defined as209,212
max
E hh
= I P+3.17U P
(2.35)
where I P is the ionization potential of the atomic gas. In the case of a few-cycles light pulses
the possibility of a single attosecond pulse generation near the cutoff photon energy has been
proposed23 and recently experimentally confirmed1.
34
Chapter 3 – Experimental Setup
Chapter 3
Experimental Setup
3.1 Descriptions of Experimental Setup
The constructed experimental apparatus for photoelectron spectroscopy in the EUV
region consists of four main parts (Fig. 3.1) :
•
femtosecond high-power laser system (0.2 TW)45
•
conversion chamber for the high harmonic generation45
•
EUV multilayer monochromator with low GVD46
•
UHV analysis chamber
35
E=15mJ@50Hz
λ=800nm
t=70fs
f=50 cm
330 l/s
high harmonic
generation
delay
stage
Ne
ML
(r=1m)
MCP+
phosphor
500 l/s
ML
XUV
photodiode
+ Al-filter
210 l/s
CCD
camera
TOF
LEED
QMA
AES
Pt(110)
UHV analysis
chamber
Xe, He
MSP+
quad-photocathode
1000 l/s
Fig. 3.1 Layout of the built experimental apparatus
36
In the framework of this theses a femtosecond laser system based on the CPA technique
has been built45. A scheme of the system is shown in Fig. 3.2., a detailed true-to-scale
technical drawing is depicted in Fig. 3.4.
master
oscillator
35 fs
5 nJ
220 ps
3 nJ
stretcher
multipass
amplifier
220 ps
1 mJ
regenerative
amplifier
compressor
220 ps
20 mJ
70 fs
15 mJ
Fig. 3.2 Scheme of the 0.2 TW femtosecond laser system
The master-oscillator – a KLM Ti:Sapphire femtosecond oscillator190 pumped with 3.6 W
from a diode-pumped frequency-doubled solid-state laser (Spectra-Physics Millenia) - serves
as a seed laser with p-polarized pulses of 5 nJ energy at a rate of 77 MHz , a bandwidth of
20 nm (FWHM) at a center wavelength of 800 nm, and a pulse duration of 35 fs (Fourierlimited pulse). The pulse spectrum is monitored with a compact Czerny-Turner type
spectrometer45. The pulse duration can be measured with a scanning second harmonic
autocorrelator45. Complete characterization of the oscillator pulses can be performed with the
single-shot second harmonic frequency-resolved optical gating (SHG-FROG) technique263,
which is presently in the test stage. Operation in the mode-locking regime is started with a
slight reversible disalignment of the laser cavity. This can be observed as a sudden change in
the spectrum of the laser pulses. The single line of the continuous mode spectrum is replaced
with a broadband spectrum of nearly gaussian distribution being a signature of the pulsed
mode. Once in the pulsed regime, the oscillator runs stable over more than 12 hours with a
slight shift of the spectral maximum of the order of 1-2 nm. Maintenance of the oscillator
rests upon a small adjustment of the oscillator end-mirror approximately every two months.
Femtosecond pulses from the master oscillator enter the pulse stretcher before amplification.
Due to high positive GVD the femtosecond pulse duration after passing the stretcher is
extended to approximately 220 ps at an energy of 3 nJ. The all-reflective pulse stretcher
working in the Littrow configuration265 has been designed with particular attention to
minimize space requirements. Changing the angle of the stretcher grating also
pre-compensates TOD of the following amplification stages. The pulse stretcher is
maintenance-free and only precise alignment of the laser beam into the stretcher is required.
37
The chirped pulses leaving the pulse stretcher are propagating through the polarizer, quartz
rotator and Faraday rotator so that the polarization is changed from p to s-polarization.
pre-pulse : pulse = 1 : 85
12.9 ns
Fig. 3.3 Main pulse and pre-pulses of the femtosecond CPA laser system
The regenerative amplifier cavity provides high quality only for p-polarized light pulses
whereas s-polarized light pulses are rejected. The Pockels-cell - synchronized with the pump
laser and master oscillator - changes the polarization from s to p for a single laser pulse in the
regenerative cavity. The confined laser pulse propagates in the cavity and becomes amplified
in the Ti:Sapphire crystal pumped with 27 mJ from a frequency-doubled pulsed Nd:YAG
laser (λ=532 nm, Spectra Physics LAB-150-50) at 50 Hz repetition rate. After the pulse
amplification saturates, the pulse propagates additional two or three round-trips in the
regenerative cavity before the Pockels-cell changes its polarization back to s and the pulse is
ejected from the regenerative cavity. The additional round-trips after the saturation are aimed
to stabilize pulse-to-pulse energy fluctuations. The amplified pulse propagates back through
the Faraday and quartz rotator but in this propagation direction the pulse polarization is not
changed and the polarizer redirects the amplified pulse to the multipass amplifier with a pulse
energy of about 1.7 mJ. The maintenance of the regenerative amplifier consists mainly of the
compensation of a small day-to-day “beam-walk” of the pump laser and tilting of the
Pockels-cell. The pre-amplified pulse from the regenerative amplifier propagates in five
successive passes through the Ti:Sapphire crystal pumped with 100 mJ from the Nd:YAG
38
laser. The multipass amplifier boosts the pulse energy up to 20 mJ. The cavity-free design of
the multipass amplifier causes its high sensitivity to an optical misalignment. As a result, a
complete new alignment of all five passes is necessary once every three months. The last
component of the CPA system is an all-reflective pulse compressor working in Littrow
configuration with an easily adjustable net negative GVD. After propagation through the
pulse compressor the laser pulse has an energy of 15 mJ and a pulse duration about of 70 fs.
The less then 100% efficiency of the polarizer in the regenerative cavity gives rise to a
leakage of pre-pulses during the amplification. The main pulse and its pre-pulses are shown in
Fig. 3.3. The pre-pulse/pulse contrast ratio of 1:85 is typical for normal day-to-day operation.
The pulse duration is minimized by means of maximalization of the pulse spectral broadening
by the supercontinuum generation266,267 after focusing the pulse in the air. The whole laser
system is situated in a separate air-conditioned room with laminar flow-boxes above the laser
bench guaranteeing a particle-free atmosphere.
A detailed down-scaled drawing of the setup for pump-probe experiments is shown in
Fig. 3.5. The laser beam is divided with beamsplitter into a pump and a probe path with an
energy ratio of 3:7. The probe beam is magnified two times with an off-axis reflective Galileo
telescope and focused with a lens (f = 500 mm) into the conversion chamber to an intensity of
1015 W/cm2 in front of the 0.8 mm diameter nozzle of a pulse valve (Lasertechnics LPV 300)
backed with 6 bar of Ne gas. Due to synchronized pulsed operation with an open duration of
145 µs a 330 l/s turbomolecular pump is sufficient to maintain a pressure of 4x10-4 mbar in
the conversion chamber. A 100 nm thin Al filter (Lebow company) as an entrance window to
the monochromator chamber reflects the fundamental and low harmonic pulses and acts as a
bandpass filter (T ~ 60%) for high harmonics with the energies between 15 eV and 73 eV.
The Al filter is placed on a motorized arm and can be moved in and out of the high harmonics
propagation path according to requirements. Under operating conditions the monochromator
vacuum chamber, pumped by a 500 l/s turbomolecular pump maintains a pressure of
1x10-6 mbar. A microchannel plate (MCP) intensified phosphor screen can be inserted into the
beam path at distance of 1 m from the pulsed valve for visualizing the high harmonics spatial
profile. The divergence (full angle) of all high harmonics within the Al-bandpass was
measured to be less than 10 mrad. For small fundamental laser energies the high harmonics
emission is very directional with a small divergence as can be seen in Fig. 3.6. The
conversion efficiency saturates at about 5 mJ of the fundamental laser energy.
stretcher
compressor
multipass amp.
Fig. 3.4 True-to-scale technical drawing of the 0.2 TW CPA femtosecond laser system
100 mm
to experiment
master oscillator
regenerative amp.
39
TOF
differential
pump-stage
300 mm
conversion
chamber
Fig. 3.5 True-to-scale technical sketch of the visible / EUV pump-probe experiment
target
UHV-chamber
multilayer
monochromator
pump-probe
optical setup
from CPA
laser system
40
41
HHG intensity
max
50 mm
min
5 mJ
fundamental laser energy
Fig. 3.6 Spatial profile of all high harmonics passing Al filter as a function of fundamental laser
energy. For the energies higher than 5 mJ the high harmonics spatial profile deteriorates.
Higher fundamental laser energies above 5 mJ result in a spatial distortion of the high
harmonic profile as shown in Fig. 3.6 and decreased conversion efficiency as a consequence
of an exceedingly high plasma density in the interaction volume which can be easily observed
visually as an incoherent plasma radiation.
The multilayer monochromator46 has to fulfill three basic requirements :
-
select a single high harmonic order from the harmonic spectrum while preserving its
femtosecond pulse duration – the selecting element must have a small GVD
-
refocus the diverging high harmonic beam into a well defined target volume and thus
providing a high light intensity
-
conserve the propagation direction of the high harmonics
The first requirement has been realized with specially designed molybdenum/silicon
multilayer mirrors43,47,268 with 30 periods of 9.9 nm thickness and a narrow bandwidth of
about 2.9 eV (FWHM). Given the thickness of a complete multilayer structure of some
d = 300 nm one can expect the temporal broadening ∆τ of the pulses not to exceed the time
delay between reflection from the multilayer surface and the substrate, respectively
∆τ < 2d (c ⋅ cosθ ) ≈ 2 fs where c is the light velocity and θ is the light incident angle. The
calculations269 of the broadening based on the Fresnel-equations actually yields ∆τ < 1 fs for
a 70 fs EUV pulse.
The second and the third requirement is fulfilled with the Z-fold mirror design where
the first multilayer is deposited on a planar substrate and the second one on a concave
spherical substrate with a radius of 1 m. The wavelength selective reflection of the multilayer
mirrors is ruled by the Bragg condition270,271. By changing the angle of incidence I can select
42
a narrow energy band and focus it to an area of ~ 1 mm2 in the UHV experiment chamber. A
variable position of both mirrors along the beam direction accomplishes a stable focus
location independent on the chosen angle of the multilayers. The currently used multilayer
mirrors are tailored for effectively selecting the high harmonics of orders 43, 45, and 47. By
changing the set of the multilayer mirrors the tuning range of the monochromator can be
extended. A EUV photodiode (XUV-100C, UDT Sensors Inc.) provides routinely control of
the photon flux available in the target which is typically 104 photons per laser shot at an
energy of 70 eV. The overall transmittance of the multilayer monochromator including Al
filter is estimated to be 5% for a single harmonic order.
The pump beam propagates through a delay stage with a translatory precision of
1.2 µm (~ 4 fs) and is then imaged onto a BBO crystal (thickness 0.3 mm) with an off-axis
reflective Galileo telescope demagnified by a factor of 2.5. The hygroscopic BBO crystal is
located in a vacuum steel chamber with Brewster-cut windows to avoid surface deterioration.
The second harmonic at 3.1 eV (λ = 400 nm) generated in the BBO crystal is focused with a
lens (f = 4 m) into the monochromator chamber where two additional mirrors are redirecting
the pump beam into the UHV analysis chamber. All mirrors after frequency doubling are
wavelength selective for 3.1 eV light energy. I have also performed experiments with a pump
photon energy of 1.55 eV (λ = 800 nm). It this case the demagnifying optics and the BBO
crystal have been removed and all the mirrors after the BBO crystal have been exchanged
according to the fundamental light energy.
The radiation of a selected high harmonic probe pulse together with a temporally
delayed pump pulse are spatially overlapped at an angle of 12 mrad onto a target in the UHV
chamber. The differential pumping stage bridging the pressure difference between the
monochromator and the experiment chamber consists of two apertures and a 210 l/s
turbomolecular pump. On demand, removing the Al filter in the monochromator and moving
a bending mirror into the pump and probe path allows an imaging of the interference pattern
of both pump and probe pulses on a CCD camera for in situ calibration of the time delay.
With this technique I can set equal optical paths for pump and probe pulses. In this case to
avoid multilayer degradation I use only unamplified femtosecond pulses directly from the
master oscillator which provides me also with a better signal statistics due to its high
repetition rate. When the second harmonic is used as pump pulse the additional BBO crystal
(thickness 0.1 mm) is introduced into the optical path of probe pulses. Correspondingly, a
blocking filter for the fundamental light is placed in front of the CCD camera and only
interference on the second harmonic is observed. After finding equal optical lengths for pump
43
and probe paths I have to correct this value by the optical thickness of the BBO crystal with
respect to vacuum (~ 166 fs). Finally the Al filter is moved back to its original position and
despite small readjustments required before running the pump-probe experiment the
pump-probe temporal coincidence is stable within about 100 fs for a few days.
pump-probe
interference
∆t = 0 fs
Fig. 3.7 Pump-probe interference pattern in case of temporal overlap and measured E-field
autocorrelation function at 1.55 eV photon energy
The observation of the interference pattern of pump and probe pulse is also informing about
possible wave-front deformations and the influence of dispersive optical elements in the
optical paths deteriorating the interference contrast. In Fig. 3.7 the interference of pump and
probe pulses with photon energy 1.55 eV is shown. A linear variation of the interference
maximum-to-maximum spacing is giving information on the wave-front tilt. This is to be
expected because of the astigmatic optical setup of the multilayer mirrors§ in the
monochromator. Graph in Fig. 3.7 shows the mean value of the interference pattern
modulation for a horizontal line-scan in the middle of the interference spot as a function of
time-delay between pump and probe pulses. Assuming two identical gaussian Fourier-limited
pump
and
probe
pulses
with
an
intensity
profile
defined
as
I (t ) = E (t ) ⋅ E (t )* = I 0 exp(−4 ln 2 ⋅ t 2 / τ 2 ) , the acquired pump-probe coherence function Γ(t )
representing the E-field autocorrelation is given as272,273
§
The calculated normal reflectivity62 of the topmost Si layer of the multilayer mirror is 35% at 800 nm and 47%
at 400 nm light wavelength.
44
+∞
Γ(t ) = ∫ E (ς ) * ⋅ E (ς − t )dς ≈ exp(−4 ln 2 ⋅ t 2 /(2τ ) 2 )
(3.1)
−∞
yielding the relation τ a = 2τ where τ a (FWHM) is the measured width of the autocorrelation
function Γ(t ) . The measured autocorrelation width of τ a = 94 fs then yields a pulse duration
of τ = 47 fs, which matches the pulse duration of 45 fs determined with the SHG
autocorrelator quite well. This result ascertains that both pump and probe optical paths do not
contain optical elements with considerable dispersion. Also the astigmatism resulting from the
monochromator mirrors setup is not critical for pump-probe experiments.
HHG intensity
100
ML
45. HH
ML
47. HH
43. HH
0
Fig. 3.8 Selectivity of the multilayer monochromator vs. photon energy of the high harmonics and
the angle of incidence of the multilayer mirrors The inset shows monochromator layout
The UHV experimental chamber has a base pressure of 3x10-10 mbar rising to
9x10-10 mbar when operating the Ne gas jet in the conversion chamber and is equipped with
the standard surface preparation equipment and analysis methods such as an ion bombardment
gun, low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and
quadrupole mass analyzer (QMA). As targets solid samples as well as gases can be
investigated. The position of the selected high harmonic within the chamber is controlled with
a quadrant metal photocathode intensified with a microsphere plate (MSP). In gas phase
experiments the low 50 Hz repetition rate of the laser system enables usage of the
45
synchronized pulsed valve providing a high local target gas density in front of the electron
spectrometer at low background pressure (10-6 mbar). Due to an unfavorable duty cycle of the
piezo-driven pulsed valve, such technique becomes less efficient with high repetition rate
(kHz) laser sources. For energy analysis of photoelectrons or Auger electrons excited with the
high harmonic pulses a time-of-flight (TOF) electron spectrometer is used45. A benefit of the
TOF technique is its capability to simultaneously analyze electrons of different kinetic
energies. An electrostatic lens system at the entrance of the µ-metal shielded electron drifttube (length 40 cm) lets me dynamically modify the acceptance solid angle of the TOF
spectrometer from 0.06% to 5% of 4π at the expense of energy resolution. To achieve better
resolution at electron kinetic energies >20 eV a retardation voltage has been applied. The
time-resolved electron signal detected on a 40 mm diameter MSP detector is transmitted to a
discriminator (Philips Scientific, Model 6904) followed by a fast multiscaler (FAST, Model
7886) with 0.5 ns time resolution. A digitizing oscilloscope (Tektronix DSA 602) proved to
be useful for alignment purposes.
As mentioned, the main challenge in the selection of a single harmonic order from the
high harmonics spectrum driven by 1.55 eV femtosecond pulses is to achieve sufficient
contrast between the desired high harmonic and the neighboring odd orders. I have used
photoionization of He gas as a precise and convenient method to measure the selectivity of
the multilayer monochromator45,46. If only one particular high harmonic is selected, only a
single photoelectron peak with the energy hν − I P is observed in photoelectron spectrum,
where hν is the photon energy and I P = 24.6 eV is the first ionization potential283 of He.
Hence, the photoelectron spectrum of He gas directly reflects the spectral distribution of the
exciting radiation. The measured selectivity of the monochromator for systematically varied
angles of the multilayer mirrors is shown in Fig. 3.8. By changing the multilayer angle of
incidence between 11 and 26 degrees selection of one high harmonic with a photon energy
between 66 eV and 73 eV has been performed. A strong absorption274 at the Al L2,3 -edge
leads to the small observed intensity of the high harmonic at 73 eV photon energy. The
highest selectivity has been achieved for 70 eV photon energy corresponding to an angle of
20 degrees. The contrast ratio to the neighboring high harmonics at 70 eV is measured to be
better than 1:10. Recently, it has been shown that the yield of one specific high harmonic can
be optimized through the variation of the driving pulse phase255,276. Applying this technique275
would enhance the contrast ratio between the selected and adjacent high harmonics.
The temporal stability of the 45th high harmonic measured as total photoelectron yield
from GaAs target is shown in Fig. 3.9. It depends on the laser system stability which is a
46
function of the pump laser pointing stability. Summary of the femtosecond CPA laser system
and the high harmonic femtosecond EUV source is given in Tab. 3.1. Detailed description of
the photon flux measurement at 70 eV photon energy and the estimation of the EUV pulse
spectral bandwidth is given in Ref. 45. The EUV pulse duration of 70 fs has been measured
by means of time-resolved photoelectron spectroscopy on Pt surface (see section 5.2). The
EUV focus size before the entry aperture of the TOF electron spectrometer has been measured
with the knife-edge technique (Appendix A). The polarization of high harmonics243 is the
same as the polarization of driving laser.
Fig. 3.9 Measured temporal stability of the 45th high harmonic over four hours
Table 3.1 Summary of the properties of the CPA fs-laser system and fs-EUV source.
Chapter 4 – Test Measurements
47
Chapter 4
Test Measurements
4.1 Photoelectron Spectroscopy on Metals, Semiconductors,
Adsorbates and Gases
High harmonics proved to be efficient tools for the static photoelectron spectroscopy
although the repetition rate of the high harmonic sources is 105 times smaller than that of the
storage rings. The first application of high harmonics as a compact table-top equivalent of
storage rings in the EUV region was demonstrated by Haight et al.120,121 Availability and easy
operation of the high harmonic sources279 made systematic and extremely time consuming
studies possible. This has lead to detailed studies of the electronic structure of metals and their
adsorbates277,278.
To illustrate the capability of my system to obtain well resolved photoelectron spectra,
I have recorded the photoelectron spectrum from the Pt(110) surface shown in Fig. 4.1. The Pt
crystal has been cleaned with acetone before transfer to the UHV vacuum chamber. In
vacuum it has been cleaned with the Ar+ ion bombardment (1.6 keV), heating (630 K) in an
oxygen atmosphere (8x10-8 mbar) and flashing (1000 K). The cleanness has been confirmed
with an Auger electron spectrum which was free of all contamination (Appendix B –
Fig. B.1). The proper surface reconstruction has been verified with LEED which displayed
well ordered (1x2) surface reconstruction280,281.
48
Fig. 4.1 Photoelectron spectrum of the Pt(110) crystal excited with the 45th high harmonic.
Fig. 4.2 Photoelectron spectrum of the p-GaAs(100) crystal excited with the 45th high harmonic
49
The photoelectron spectrum excited with the high harmonic of 70 eV photon energy shows a
dominant and well resolved Pt valence band282. In order to increase the energy resolution of
the TOF spectrometer the retarding voltage† of 35 V has been used. The light of 70 eV photon
energy generates core-holes283,284 in the 5p3/2 (Ebin = -51.7 eV) and 5p1/2. (Ebin = -65.3 eV)
states. The kinetic energy§ of the 5p3/2 photoline excited with the 70 eV photon energy is only
12.9 eV and has not been detected with the used retarding voltage. The photoemission from
the 5p3/2 state with 70 eV photon energy has enhanced cross section (see equation (2.4)) due
to the high density of final states278,285 between 18 eV and 20 eV above the Fermi niveau. The
O3VV Auger electrons (Fig. 4.1) resulting from the 5p3/2 core-hole decay are therefore well
pronounced278,286-288.
The photoelectron spectrum of the p-GaAs(100) crystal is shown in Fig. 4.2. The
retarding voltage of 5 V has been used. The GaAs crystal has been cleaned290 in toluene,
acetone, tricholoethylene, acetone and ethanol before transfer into vacuum. In vacuum a clean
p-GaAs(100) surface has been achieved with repeated cycles of sputtering with 1.5 keV Ar+
ions and annealing at 500 °C. After such preparation the Auger electron spectra showed no
carbon and oxides contamination (Appendix B – Fig. B.3). The GaAs(100) surface exhibits a
variety of reconstructions which are conveniently recognized observing the photoline
intensity ratio Ga-3d/As-3d136,295-297. The photoelectron spectrum clearly resolve Ga-3d and
As-3d photolines. The Ga-3d photoemission cross section291,292 has a maximum for 70 eV
photon energy and the photoelectrons emitted from the surface exhibit maximum surface
sensitivity68. The inset of the Fig. 4.2 shows the As-3d resolved spin-orbit splitting293,294 of
0.7 eV. The photoemission cross section for the GaAs valence band at 70 eV photon energy is
more than one order of magnitude smaller as compared to the Ga-3d cross section. The
measured selectivity ratio of the 45th to 47th high harmonic is about 6%. This can be further
reduced to a virtually pure single high harmonic under the special circumstances shown later
in Fig. 6.4.
To demonstrate the capability of the built apparatus also the photoelectron spectra of
adsorbates have been measured. Fig. 4.3(b) shows the photoelectron spectrum of
wolframhexacarbonyl W(CO)6 adsorbed on a Si(100) surface together with the W(110)
photoelectron spectrum (Fig. 4.3(a)) for comparison. The TOF retarding voltage of 0 V has
been used. The W(110) crystal has been cleaned by several flashing and oxidation cycles.
†
The kinetic energy of electron throughout this thesis is refereed to the electron’s kinetic energy direct after
emission from the target and not after the retardation in the TOF spectrometer.
§
The work function289 of Pt(110) (1x2) surface is 5.4 eV.
50
Fig. 4.3 Photoelectron spectra of the W(110) crystal (a) and W(CO)6 adsorbate (b)
N
O
N
Ga
O
N
O
He I - UPS
Fig. 4.4 Photoelectron spectrum of Gaq3. The UPS spectrum is taken from Ref. 307.
51
Surface cleanness has been confirmed by the observation of well pronounced 4f5/2 and 4f7/2
photolines in the photoelectron spectrum (Fig. 4.3(a)) which are surface sensitive. On the
basis of known values of the binding energies283,301-303 for 4f5/2 (Ebin = -33.6 eV) and 4f7/2
(Ebin = -31.4 eV) states the photoelectron spectrum has been converted to the electrons’
binding energies to make the direct comparison with W(CO)6 possible. The W 4f binding
energies in Ref. 283 are given with the accuracy of ±0.1 eV which sets the upper limit on the
chemical shift error evaluation although the peak position for the W 4f5/2 is determined by the
fitting with Gauss function with an error of 10 meV. The W(CO)6 has been evaporated onto
Si(100) surface terminated with an OTS (Octadecyltrichlorosilane) self-assembled
monolayer305 with the Si(100) substrate held on -100°C with a liquid nitrogen cooler. After
evaporation the W(CO)6 adsorbed layer could be observed visually as the reflectivity of
substrate had changed. The adsorbed W(CO)6 photoelectron spectrum (Fig. 4.3(b)) shows
well resolved molecular orbitals belonging to the CO groups298-300 as well as both core-level
4f lines. The photoelectron spectrum has been converted to the electrons’ binding energies on
the knowledge of the CO groups position298. The distinctive chemical shift304 ∆EEC = 1.2 eV
of 4f core-levels points on the different electronic environment of the W atoms in the W(110)
surface and the W(CO)6 adsorbate, respectively. This demonstrates the usefulness of this
apparatus for electron spectroscopy for chemical analysis (ESCA) in the EUV region.
Recently the technical relevance of the organic materials306 for light-emitting diodes
was recognized. One of the promising and extensively studied materials is tris(8hydroxyquinolinato) gallium (Gaq3)307-313. Fig. 4.4 shows the photoelectron spectrum of Gaq3.
The TOF retarding voltage of 30 V has been applied. The Si(100) crystal has been stripped of
its natural oxide layer in a buffered HF solution and transferred into the vacuum chamber
where it was cleaned with heating cycles up to 830 °C. For the evaporation314 of Gaq3 a Al2O3
single-ended tube (Friatec AG) resistively heated with molybdenum wire has been used at
temperatures up to 280 °C. The presence of a Gaq3 film has been checked as a visual change
in the substrate reflectivity. The photoelectron spectrum in Fig. 4.4 shows the well resolved
Gaq3 molecular orbitals excited with 70 eV photon energy together with a spectrum measured
with He I-UPS for comparison307. The Ga-3d core-level photoline in Fig. 4.4 points to the
excellent sensitivity of this measurement method for a single atomic species embedded in a
large molecular complexes.
52
Fig. 4.5 Photoelectron spectrum of I2 adsorbate excited with the 45th high harmonic after
subtraction of the secondary electron background. The inset shows the originally measured
spectrum.
10
Xe
8
hν = 73 eV
5p1/2 5p3/2
5s1/2
6
4
intensity (a.u.)
10
N4,5O2,3O2,3-Auger
2
0
25 23 21 19 17 15 13 11
binding energy (eV)
8
6
5s-Satellites 5s
4
5p
2
0
30
35
40
45
50
55
60
kinetic energy (eV)
Fig. 4.6 Photoelectron and Auger spectrum of Xe after photoionization with 73 eV.
9
53
I have also investigated the photoelectron spectra of iodine films with the objective of
Auger 4d core-hole decay observation. Iodine films have been prepared by condensing I2
molecules from an electrochemical AgI cell315,316 onto Si(100) surface terminated with OTS
self-assembled monolayer cooled with liquid nitrogen to –100 °C. The photoelectron
spectrum of a I2 adsorbate excited with 70 eV photon energy is shown in Fig. 4.5. The TOF
retarding voltage of 0 V has been used. The iodine 4d3/2 and 4d5/2 photolines are obscured
with a massive secondary electron background as shown in inset of Fig. 4.5.
Double-exponential background subtraction reveals both 4d core-level photolines. The
measured spin-orbit splitting317 is ∆EFS = 1.7 eV. The N4,5O2,3O2,3 Auger decay of the 4d
core-hole is observed as a broad less prominent feature318 between 20 eV and 30 eV kinetic
energy.
The first measurements45, however, were performed in the gas phase photoionizating
He for spectral characterization of the multilayer monochromator (see section 3.1). For
completeness, Fig. 4.6 shows the photoelectron and Auger spectra of Xe gas after the
photoionization with 73 eV photon energy and TOF retarding voltage of 15 V. The
photoionization of the 5s and 5p shells gives rise to the photoelectron peaks at energies
49.5 eV and 61 eV, respectively. Increasing the retardation voltage of the TOF spectrometer
to 35 V and using the photon energy of 70 eV, I can observe the spin-orbit splitting of the 5p
energy level (inset in Fig. 4.6). The photon energy of 73 eV is sufficient to create a hole state
in the 4d shell which relaxes through the characteristic NOO Auger decay. Electrons
corresponding to the N4,5O2,3O2,3 Auger decay as well as 5s-satellites are well distinguished in
the electron spectrum. The measured linewidth of 0.9 eV (FWHM) of the Auger electron peak
N5O2,3O2,3 (1S0) is considerably larger than the natural linewidth319 of 120 meV. Since Auger
linewidths are generally independent on the bandwidth of the exciting radiation, this value
represents the energy resolution of the TOF spectrometer at 30 eV electrons’ kinetic energy
and 15 V retardation potential. The variation of the TOF energy resolution has been analyzed
in Ref. 45 in detail. Taking into account the limited resolving power, the photoelectron and
Auger electron spectrum are in reasonable agreement with spectra measured with synchrotron
radiation320,321.
54
Chapter 5 – Time-Resolved Photoelectron Spectroscopy on Pt(110) Surface
Chapter 5
Time-Resolved
Photoelectron
Spectroscopy
on
Pt(110) Surface
5.1 Measurement of Hot Electrons on a Pt(110) Surface
The idea of the experiment is based on the photoexcitation of electrons from occupied
states of the valence band into unoccupied states above the Fermi level with a visible pump
pulse. These excited short-lived electrons (see section 2.1.2) (also called “hot electrons”) are
then emitted with an EUV probe pulse into vacuum where they are energy-analyzed. The time
delay between pump and probe pulses can be continuously changed. In this way, I obtain
information on the transient population above the Fermi level at the surface. The
implementation of the principle for the Pt(110) surface is shown in Fig. 5.1. The pump pulse
with 1.5 eV photon energy and 70 fs duration excites electrons from occupied d-band states322
in the valence band into unoccupied states above the Fermi level as shown in the energy-time
representation in Fig. 5.1(a).
Fig. 5.1 Principle of the visible-pump/EUV-probe hot electron spectroscopy on a Pt surface
shown in (a) energy-time representation and (b) space-time representation
55
56
The total absorbed light intensity of 1.6x1011 W/cm2 excites 2.2% of all valence band
electrons§. These hot electrons relax very rapidly via e-e scattering. It has been shown89 that
the hot electron’s life times of transition metals are remarkably shorter than the life times of
noble metals. This phenomenon is due to the large phase space for e-e scattering caused by
the high density of states322 (d-band) at the Fermi edge. On the contrary, totally filled d-bands
of the noble metals and solely contribution of the s-bands to the e-e scattering at the Fermi
edge are responsible for the relatively long-lived hot electrons. The probe pulse with a photon
energy of 70 eV and duration of less a 70 fs projects the current electron distribution into the
vacuum as shown in Fig. 5.1(a). Changing the time delay between pump and probe pulses I
can track the temporal evolution of the electronic population at the Fermi edge. Here I define
the time delay as positive when the pump pulse precedes the probe pulse. The kinetic energy
of the hot electrons emitted with the probe pulse is about 65 eV. Such electrons are mainly
(86% of all registered electrons) originating326 from only 0.88 nm depth. At this point I have
to assume also a ballistic transport (1 nm/fs) and a diffusion (10 nm/ps) of hot electrons103
from the probe spot at the surface (Fig. 5.1(b)). Consequently, the net observed relaxation
times of hot electrons at Pt surface is expected to be smaller than 3 fs for the hot electrons of
1.5 eV kinetic energy with respect to the Fermi niveau.
I have used a variant of the experimental setup shown in Fig. 3.1 without the
frequency doubling unit for pump pulses. In this experiment the repetition rate of the laser
system was 10 Hz and the time-resolution of used TOF electron detection system was 1.5 ns.
The clean Pt(110) surface has been prepared as described in section 4.1. The p-polarised
pump pulses (800 nm, 1.55 eV) with energy 0.5 mJ and duration of 70 fs were focused
together with the 70 eV p-polarised probe pulses on the Pt(110) surface. The pump and probe
incident angle measured from the surface normal was 53°. The axis of the TOF spectrometer
and surface normal made an angle of 8° and the retardation voltage was 35 V. The
photoelectron spectra for –400 fs, 0 fs, and +400 fs delay are shown in Fig. 5.2. The
photoelectron spectra for ±400 fs are identical in the region above the Fermi level indicating
that hot electrons for +400 fs are complete thermalized. The photoelectron spectrum for time
delay 0 fs shows a clear deviation when compared with the spectra for ±400 fs. When pump
§
Total number of absorbed photons NA = 8.4x1014 is given by NA = (1-R)NI where NI=2x1015 is the number of
incident photons and R = 0.58 is the reflectivity62 of Pt for the incident angle 53° and photon energy 1.55 eV.
The 86% of all photons are absorbed62 in the depth of 26 nm. The pump spot at the surface was 19x10-3 cm2,
resulting in total excited electron density of 1.5x1022 cm-3. The electron density61 of Pt valence band (5d, 6s
electrons) is 6.6x1023 cm-3, yielding the net valence band electronic excitation of 2.2%.
57
and probe pulses hit the Pt surface in temporal coincidence I have to observe the highly
non-equilibrium electron distribution above the Fermi edge. This has been observed as shown
in the inset of Fig. 5.2. The probe pulse with 70 eV photon energy creates a core-hole in the
5p3/2 niveau. This core-hole relaxes as shown in section 4.1 via Auger decay seen as a O3VV
Auger peak on the top of a secondary electron background. The relation between the O3VV
peak shape and the valence band density of states (DOS) is not elementary and in the first
approximation is given by the self-convolution of the valence DOS327-330.
Fig. 5.2 Photoelectron and Auger electron spectra of Pt(110) surface for 0 fs and ±400 fs delays
between pump and probe pulses.
58
Fig. 5.3 Fermi edge photoelectron spectra of the Pt(110) surface for time delays of 0 fs and
±100 fs between pump and probe pulses.
Naively, one would expect a simple broadening of the O3VV peak due to new possibilities in
the Auger recombination process connected with the transient hot electron distribution in the
normally unoccupied states above the Fermi level. As shown in the inset of Fig. 5.2, the
O3VV Auger peak shifts to lower kinetic energies. A detailed theoretical study of this
phenomenon is necessary to account for the observed O3VV Auger peak shift. In the
following I will concentrate exclusively on the hot electron dynamics at the Fermi edge which
gives more transparent and easily interpretable results. More detailed photoelectron spectra
for the three delay times 0 fs and ±100 fs at the Pt Fermi edge measured with the retardation
voltage of 45 V are shown in Fig. 5.3. A considerable non-equilibrium electron population
above the Fermi edge is present at zero time delay. The spectrum for +100 fs shows no signs
of hot electrons above the Fermi edge revealing a very fast hot electron thermalization as
expected. The inset of the Fig. 5.3 shows the photoelectron spectrum generated exclusively
with the pump pulses which can be attributed to multiphoton emission processes331 at such
high pump intensity. Only by using probe photons with energies exceeding 40 eV the
photoelectron signal can be prevented from being obscured by these background electrons. I
attribute the observed time-dependent spectral variation of the photoelectron signal at the Pt
Fermi edge to a transient hot electron population rather than to a space-charge broadening
59
induced by the high local electron density because i) the distinct spectral feature observed at
65.5 eV in Fig. 5.3 does not indicate a simple broadening of the Fermi-edge and ii) due to a
velocity of <3.5 µm/ps for electrons with energies lower than 35 eV I would not expect an
ultrafast space charge relaxation. A more detailed discussion on the space-charge phenomena
is given in section 6.1.
pump hν = 70 eV
probe hν = 1.5 eV
EF + 70 eV
65.5 eV
probe
pump
pump
probe
Fig. 5.4 Contour plot of the photoelectron yield vs. kinetic energy and time delay between
pump and probe pulses measured at the Pt(110) surface. The time-resolved signal at
photon energy 65.5 eV (dashed line) is shown in Fig. 5.8.
A contour plot of the photoelectron yield vs. kinetic energy and time delay between pump and
probe pulses (Fig. 5.4) exhibits a fast relaxation of the hot electrons within 100 fs. The data
were acquired in four subsequent runs with 96 min duration, each. During this time, no
significant changes in the temporal structure of the spectra has been observed. For large
negative time delays (<-100 fs) when the probe pulse precedes the pump pulse the measured
photoelectron spectra reflect the non-disturbed Fermi edge. For large positive delays
(>+100 fs) the hot electron population has already decayed and the measured photoelectron
spectra are same above the Fermi edge as the spectra for large negative time delays. For time
delays between ±100 fs I observe a symmetrical hot electron distribution above the Fermi
60
edge with respect to zero time delay. When I assume the temporal profile to be a delta-like
function for pump and probe pulses one has to observe the exponentially decreasing hot
electron distribution for positive time delays with a maximum for zero time delay. For
negative time delays there should be no hot electron population. Consequently, this
pump-probe method is not symmetrical for time delay inversion. The actually measured
symmetrical distribution for time delays between ±100 fs is caused by the finite time duration
of pump and probe pulses which is considerably larger than the characteristic hot electrons
life times (see equation (2.27)). This means that the measured hot electron distribution is
proportional to the cross-correlation between pump and probe.
Fig. 5.5 Fermi edge time-resolved photoelectron spectra for the Pt(110)
The decrease of the time-resolved photoelectron yield for the electron energies lower
than 64 eV (representing the Fermi edge), correlated with the hot electron population, can be
understood as an electron depletion of the valence band. At the absorbed intensity of
1.6x1011 W/cm2 I deal with a photoexcitation of 2.2% of all valence band electrons. Even
larger photoexcitation can lead to the short time scale destabilization of the crystal
structure324,325. This in turn can be observed as an alteration of the metal dielectric
function333-335. There are also theoretical studies336,337 for semiconductors where the
photoexcitation of 10% of all valence band electrons leads to collapse of the electronic
structure338-348 observed as semiconductor band-gap collapse, semiconductor-to-metal
61
transition, and structural changes leading to an ultra-fast femtosecond melting of the
semiconductor. This is the first direct study of the hot electron population and valence band
changes at such high pump intensities. The traditional studies79 have been performed at
intensities more than 10 times lower than in this case. The used pump intensity in my study is
still considerably lower than the threshold for plasma formation at metal surfaces332 of
1013 W/cm2.
before
photoexcitation
after
photoexcitation
EF
Fig. 5.6 Photoelectron spectra near the Pt Fermi edge before and after photoexcitation
Upgrading of the laser system to a repetition rate of 50 Hz has lead to a faster data
acquisition. Additionally, a better time-resolution of the TOF electron detection system of
0.5 ns enabled to do the same experiment on the Pt(110) surface with shorter accumulation
times and better photoelectron energy resolution. The measured time-resolved photoelectron
spectra are shown in Fig. 5.5. The shown spectra have been measured in two sequential runs,
each of 52 min duration. The Fermi edge at zero time delay shows the highly non-equilibrium
hot electron distribution. The e-e scattering is leading to the hot electron thermalization within
few femtoseconds. However e-p interactions occur on a time scale up to a few picoseconds
leading to lattice heating (see Fig. 2.8) resulting in a less pronounced – less sharp Fermi
edge61. The photoelectron spectra showing the Fermi edge region of the Pt valence band
before and after photoexcitation are shown in Fig. 5.6. The observed difference in the
photoelectron spectra is not due to a high harmonic intensity drop off, because the shown
spectra were acquired in two sequential runs and have been normalized so that the integral of
62
all registered photoelectrons for all spectra is constant. I therefore associate the observed
difference at the Fermi edge with the growing lattice temperature after photoexcitation.
5.2 Pump-Probe Cross-Correlation in the EUV Region
The measurement of femtosecond (or even attosecond) pulse duration in the EUV
region represents a great experimental challenge. In the infrared, visible and ultraviolet
traditional techniques based on nonlinear effects are used for the pulse duration
measurement73 yielding not only amplitude, but also phase information263,264. The current
EUV pulse intensities are not high enough to make nonlinear effects observable. Accordingly,
the autocorrelation techniques based on nonlinear effects are not available. The tendency is to
use instead cross-correlation techniques with a high-intensity visible or infrared “dressing”
pulse and an EUV pulse in an experimental setup with a generalized scheme like in Fig. 5.7.
“dressing”
pulse
to spectrometer
EUV pulse
e-
time
delay
Fig. 5.7 Scheme of the “dressing-pulse” cross-correlation techniques in the EUV region
One measures photoelectron350 or Auger electron349 spectra of the noble gases as a function of
the time delay between ionizing EUV pulse and “dressing” pulse with an intensity higher than
1011 W/cm2. The photoelectron spectrum in the presence of the strong “dressing” pulse field
exhibits side-bands6,350 of photoelectron peaks as well as peak’s broadening and shifting1,7.
This technique has been successfully applied even to the measurement of the trains of
attosecond pulses6 and single attosecond pulses1 as well. Assuming chirp-free EUV pulses
one can estimate their duration also with linear effects like interference227,357-359. However,
daily usage of the femtosecond pulses for the time-resolved spectroscopy in the EUV region
is calling for a UHV compatible measurement method utilizing lower intensities than
1011 W/cm2. The cross-correlation method utilizing the ultrafast relaxing hot electrons on
63
transition metal surfaces seems to be very efficient and reliable tool for the measurement of
femtosecond EUV pulses.
In the section 5.1 I have shown the measurement of a very fast decaying hot electron
distribution at the Fermi edge of the Pt(110) surface. The measured transient signal from the
hot electron distribution does not allow extraction of the hot electron’s life-times of some few
femtoseconds but is proportional to the pump-probe cross-correlation. Let us consider hot
electrons with a kinetic energy of 1.5 eV referring to the Fermi level photoexcited with the
0
pump pulse modeled by the gaussian intensity profile I pu = I pu
exp(−4 ln 2 ⋅ t 2 / τ 2pu ) . The
photoemission of these hot electrons into vacuum with the EUV probe pulse with the intensity
0
profile defined as I pr = I pr
exp(−4 ln 2 ⋅ t 2 / τ 2pr ) results in a photoelectron signal depending on
the time delay between pump and probe pulses. The photoelectron signal at the kinetic energy
of 65.5 eV corresponding to these hot electrons is shown in Fig. 5.8. This signal corresponds
to a line-scan shown through the contour plot in Fig. 5.4. Assuming that the registered
photoelectrons are governed by the Poisson probability distribution361 the error bars in
Fig. 5.8 show the standard deviation§ of the measured photoelectron counts supposing these to
be the mean value. The evaluated FWHM error of the cross-correlation signal is given as the
standard deviation of the Gauss function width fitting parameter.
Under the approximation of a delta-like fast hot electron relaxation the measured
cross-correlation signal S (τ ) as a function of time delay τ between pump and probe pulses is
given as
+∞
S (τ ) = ∫ I pu (ς ) I pr (ς − τ )dς ≈ exp(−4 ln 2 ⋅ t 2 /(τ 2pu + τ 2pr ))
(5.1)
−∞
On the basis of this result the measured square of FWHM of the cross-correlation signal is
proportional to the sum of the pump and probe pulse duration (FWHM) squares. The
measured pulse width of the pump pulse45 of 70 fs and the cross-correlation width of 100 fs
yield the EUV probe pulse duration of approximately 70 fs neglecting the hot electron
lifetime. A spectral and spatial drift of the driving femtosecond laser and mechanical system
instabilities over the long photoelectron spectra acquisition times have been slightly reduced
§
Presumed µ is the mean of the Poisson probability distribution the standard deviation is given by
µ
64
utilizing a new pump laser of 50 Hz resulting in the shorter width of cross-correlation signal
show in Fig. 5.8(b).
An application of this method for pulse duration measurements in the EUV region has the
following advantages:
-
reduced pump intensity of one or two orders compared to the “dressing” pulse
intensity higher than 1011 W/cm2 necessary in gas-phase cross-correlation
-
UHV compatible environment during the measurement compared to the some
10-4 mbar in the “dressing” pulse cross-correlation methods
intensity (arb. units)
FWHM = 100 ± 5 fs
intensity (arb. units)
FWHM = 92 ± 7 fs
Fig. 5.8 Cross-correlation of pump and probe pulses corresponding to hot electrons with a
kinetic energy of 1.5 eV above the Fermi level as measured with (a) 10 Hz and (b) 50 Hz
laser system.
65
Until now, I have supposed an instant hot electron relaxation but remembering the finite
relaxation times of hot electrons restricts this method to measurements in the femtosecond
region. But the perspective of higher excitation energies of some 3–4 eV of the pump pulse is
leading to even shorter hot electron relaxation times which can push the applicability of this
method even to the femto-atto-second edge.
66
Chapter 6 – Time-Resolved Photoelectron Spectroscopy on GaAs(100) Surfaces
Chapter 6
Time-Resolved
Photoelectron
Spectroscopy
on
GaAs(100) Surfaces
6.1 Measurements of Transient Ga-3d Core-Level Shifts on
GaAs(100) Surfaces
The principle of the pump-probe measurement used throughout this section is shown
in Fig. 6.1. The pump pulse (3.1 eV) with an intensity of 1x1010 W/cm2 generates
approximately 1.6x1020 cm-3 electron-hole pairs§ in the SCR at the p-GaAs(100) surface. The
SCR electric field of few hundreds of kV/cm separates generated electron-hole pairs in less
than 1 ps. The electrons drifting to the surface partially compensate the SCR electric field
which is leading to the bands flattening. The core-levels362 are bending in the same manner
and with the same amplitude as conduction and valence bands. The time-delayed probe pulse
with energy of 70 eV emits the electrons from the Ga-3d core-level into vacuum.
§
Total number of absorbed photons NA = 1.25x1013 is given by NA = (1-R)NI where NI=3.14x1013 is the number
of incident photons and R = 0.6 is the reflectivity62 of GaAs for the incident angle 53° and photon energy 3.1 eV.
The 86% of all photons are absorbed62 in the depth of 30 nm. The pump spot at the surface was 23x10-3 cm2,
resulting in total excited electron-hole density of 1.6x1020 cm-3. The electron density105 in GaAs valence band is
2x1023 cm-3, yielding the net valence band electronic excitation of 0.1%.
Fig. 6.1 Principle of the band-bending after photoexcitation detected by measuring the
kinetic energy of the photoelectrons from the Ga-3d shell.
67
68
Accordingly, the kinetic energy of Ga-3d photoelectrons is shifting by the same amount as
that of valence bands. The Ga-3d photoelectrons’ shifts are therefore giving information on
the band-bending as a function of time delay between pump and probe pulses. The surface
carrier recombination leading to band-bending receding to the original value is observed as
the back-shift of the Ga-3d photoelectrons’ kinetic energy. The high excitation intensity of the
pump pulses is giving rise also to multiphoton processes seen as a low-energetic electron tail.
The kinetic energy of the Ga-3d photoelectrons emitted with photons of 70 eV energy retains
maximal surface sensitivity of circa 1 nm depth.
Fig. 6.2 Photoelectrons excited through multiphoton processes with a pump pulse (3.1 eV)
of intensity 10 GW/cm2 from the p-GaAs(100) surface
Before discussing the experimental results on transient changes in the SPV at the
GaAs surface I elucidate some particular challenges connected with the time-resolved
photoelectron spectroscopy at high pump intensities. While in my visible-pump/EUV-probe
scheme I aim for a single-photon electronic excitation of the semiconductor, the large number
of photons reaching the surface in a short time interval (about 1013 in 70 fs corresponding to
~10 GW/cm2) also leads to multiphoton emission (MPE)363,364 of energetic electrons, as
depicted in Fig. 6.2. In the case of low energetic probe photons in the visible or UV range the
strong MPE electron background will obscure the pump-probe photoelectron signal and has
therefore restricted such studies in the past to considerably lower intensities. I overcome this
69
problem by use of EUV probe photons with 70 eV energy and so I achieve sufficient energy
separation between the measured signal and the MPE low energetic electron tail becoming
confined to energies below 11 eV for 10 GW/cm2 pump intensity. However, the signal
electrons will still be affected owing to vacuum space charge156.
Fig. 6.3 Valence band photoelectron spectra of the Cu(110) with and without pump pulse
indicating the influence of vacuum space charge decoupled from the SPV effect.
In the following I will use the maximum kinetic energy - MPE cut-off - of the MPE
background electrons rather than pump intensity which depends on the spatial pump pulse
profile. The study of the copper valence band photoelectron spectra in the highly excited
regime has been selected as an example of a system which does not exhibit the SPV effects
and therefore permit an independent study of the vacuum space charge influence on the
measured photoelectron spectra without interfering SPV effect. The Cu(100) surface excited
with pump pulse of 1.55 eV photon energy producing the MPE background electrons with up
to 35 eV kinetic energy has been probed with probe pulse of 70 eV photon energy. The
Cu(100) crystal has been cleaned with repeated cycles of Ar+ sputtering (1.5 keV) and
annealing at 580 °C, in the last cycle only at 200 °C to avoid surface contamination from the
bulk. The surface cleanness has been verified by a contamination-free Auger electron
spectrum (Appendix B – Fig. B.2). The Cu valence band photoelectron spectra excited with a
probe pulse in the presence and absence of pump pulses are shown in Fig. 6.3. I have decided
70
to use the pump pulse with photon energy of only 1.5 eV to avoid direct photoexcitation from
the d-bands322 of Cu valence band and so be able to observe a well defined d-bands edge also
at high pump intensities. The Cu valence band photoelectron spectra for time delays ±100 fs
between pump and probe are identically shifted to higher kinetic energies without significant
broadening of spectral features. This can be understood as an acceleration of the leading fast
probe photoemitted electrons by Coulomb repulsion from the trailing cloud of slow MPE
pump photoemitted electrons. In the following I therefore express the measured SPV
transients in terms of a time-dependent relative shift with respect to the situation when the
probe pulse precedes the pump pulse by 800 fs (negative delay).
Evac
CBM Eth ~ 5.5 eV
VBM
valence
band
As 3d ~ 41.5 eV
Fig. 6.4 Photoelectron spectra of the p-GaAs(100) excited with a spectrally pure 45th high
harmonic. The inset shows the simplified GaAs electronic structure.
Time-dependent photoinduced changes of the band-bending were studied for p-doped
(Zn: 3x1019 cm-3) GaAs(100) as well as n-doped (Te: 6x1017 cm-3) GaAs(100) surfaces. The
clean surfaces were prepared as described in detail in section 4.1. The photoelectron spectrum
of the clean p-GaAs(100) surface is shown in logarithmic scale in Fig. 6.4. The photoemission
cross section for Ga-3d core-level electrons is nearly two orders of magnitude higher than that
for the valence band electrons. This is the reason why the currently feasible short
photoelectron accumulation times are prohibiting the direct observation of the electron
population dynamics in the conduction band or surface states as a result of insufficient
photoelectron signal statistics. Accordingly, in the further text I deal exclusively with the
71
Ga-3d core-level photoelectrons. The inset of Fig. 6.4 shows also a simplified (without
band-bending) electronic structure of the GaAs crystal. The most stunning feature of the
photoelectron spectrum in Fig. 6.4 is the absence of the adjacent high harmonics
photoelectron signal vanishing in the secondary electron background. This can be achieved by
the interplay of the high harmonics driving fundamental wavelength and the incident angles
of multilayers. This method is yielding a nearly spectrally clean single high harmonic
selection.
47.80
CBM
47.66
VBM
p-GaAs(100)
47.42
CBM
47.47
VBM
n-GaAs(100)
Fig. 6.5 Shift of the Ga-3d core-level of (a) p-GaAs and (b) n-GaAs probed 400 fs before and
after photoexcitation. The calculated peaks’ center-of-gravity are shown as vertical continuos
lines for +400 fs and vertical dashed lines for -400 fs. The insets show schematically the
photoinduced band-bendings.
72
For an energy of 270 µJ/cm2 deposited by each 3.1 eV pump pulse neither changes of the
Ga-3d core-level spectra by photochemical decomposition155 nor any macroscopic alterations§
of the GaAs surface have been observed after the pump-probe experiment with irradiation
over several hours. The deposited energy by the pump pulses is lower than the known
photochemical decomposition intensity threshold of some 20 mJ/cm2 specified for pGaAs(100) in Ref. 161 or 1 mJ/cm2 for n-GaAs(110) in Ref. 155. In Fig. 6.5(a) are presented
photoelectron spectra of the Ga-3d inner shell of the p-GaAs(100) at positive and negative
pump-probe delay. The observed transient shift of the Ga-3d photoline center-of-gravity
towards higher energies after photoexcitation (+400 fs) corresponds to a partial screening of
the electric field in the SCR by the mobile carriers created by the optical pump pulse.
According to Fig. 6.1 this in turn leads to a reduction of the effective binding energy of the
Ga-3d state at the semiconductor surface. Due to the different doping concentrations of n and
p-GaAs crystals I am not expecting the same amplitude of the shifts for these two crystals but
finding the corresponding shift for n-doped GaAs(100) at lower kinetic energies as indicated
in Fig. 6.5(b) I can exclude this effect as being induced by repulsion from MPE electrons as
discussed in the case of vacuum space charge. Rather, the opposite sign is compatible with a
reduction of band-bending towards higher effective binding energy as expected for n-doping.
Since the p-doped crystal exhibited a more pronounced transient energy shift, I focused my
investigation on the p-GaAs(100) as indicated in Fig. 6.6 as a sequence of photolines at
different visible/EUV time delays. To rule out possible adsorbates induced core-level
chemical shifts from residual gases, photochemical decomposition, and temporal drifts of the
laser or the electron detection system during the long photoelectron acquisition times the
spectra with subsequent time delays have been measured in a random fashion rather than in an
ascending order. The pump-probe spatial overlap was verified every time when the time delay
has been changed by more than 1 ps with the retractable pinhole of 1.2 mm diameter and a
CCD camera. The Fig. 6.6(a) shows the photoelectron yield as a function of time delay and
electrons’ kinetic energy. For negative time delays - which corresponds to the situation when
probe pulse precedes pump pulse - I observe no significant variations in the Ga-3d photoline.
For positive time delay smaller than 1 ps I notice a kinetic energy shift of Ga-3d
photoelectrons to higher kinetic energies.
§
The visual inspection of the used part of GaAs surface in the pump-probe experiment revealed no changes
when compared to the unexposed part of surface examined with a light microscope and magnification up to
200x.
(a)
probe
pump
pump
probe
(b)
Fig. 6.6 Transient in p-GaAs(100) revealed by the Ga-3d core-level shift as a function of the
pump-probe delay - (a) contour plot and (b) for selected time delays. The thin vertical lines in
graph (b) indicate the peaks’ center-of-gravity positions.
73
74
Photoexcitation of the electron-hole pairs in the SCR is followed by the charge separation
phase, where the electrons are drifting to surface and the holes into the bulk. This charge
transport is connected with gradual decrease of the SCR electric field and corresponding
flattening of the electronic bands as well as core-levels. This typical behavior can be seen also
in Fig. 6.6(b) for time delays +100 fs and +1 ps. For time delays longer than 1 ps I observe
slow gradual decrease of the Ga-3d photoelectrons’ kinetic energy. This phase is linked with
the surface recombination processes (see Fig. 6.6(b) for delay times +8 ps, +32 ps). The
measured recombination has nearly exponentially decaying character recovering the old value
of the band-bending approximately after +32 ps. Fig. 6.6(b) shows also Ga-3d photoline in the
absence of pump pulse. Accordingly, I observe a permanent shift with respect to the
non-pumped surface as a collective result of the band-bending induced by a possible pre-pulse
preceding the femtosecond pump pulse by 12.9 ns, of residual non-decayed carriers from the
previous pump pulse 20 ms before and of the vacuum space charge effect as discussed above.
n-GaAs(100)
pump hν = 3.1 eV
probe hν = 70 eV
(a)
SPV shift
no MPE
(b)
SPV shift
MPE
cut-off 11 eV
space-charge
shift
(c)
Fig. 6.7 Effect of the vacuum space charge created by pump pulses on the Ga-3d
photoelectrons’ kinetic energy at n-GaAs(100) surface
75
In order to understand the effects of the vacuum space charge created by pump pulses
on the Ga-3d photoelectrons I have measured the Ga-3d photoelectrons’ kinetic energy for
different pump fluences at n-GaAs(100). The SPV effect on the n-GaAs(100) surface shifts
the kinetic energy of the Ga-3d photoelectrons to lower kinetic energies. In Fig. 6.7 Ga-3d
photoelectron spectra are shown for three different cases - without pump pulses (Fig. 6.7(a)),
at the pump intensities when no vacuum space charge has been generating (Fig. 6.7(b)) and at
the pump intensities generating vacuum space charge consisting of the MPE electrons with
11 eV kinetic energy cut-off (Fig. 6.7(c)). In the regime when no vacuum space charge has
been generated the SPV effect is dominant and the Ga-3d photopeaks have lower kinetic
energy than the Ga-3d photopeak measured at the unpumped surface (Fig. 6.7(b)). In this case
the Ga-3d photopeaks 400 fs before and 400 fs after photoexcitation do not differ remarkably
in their position. It has been shown161 that the ratio of the amplitudes of fast and slow SPV
shifts are about 14%. In this case the fast transient SPV shift should be some 60 meV which
needs a better photoelectron statistics to prove it. In Fig. 6.7(c) the presence of vacuum space
charge built of the electrons with kinetic energy cut-off of 11 eV is shifting the Ga-3d
photopeaks to higher kinetic energies. The position of the Ga-3d photopeaks in this case is
nearly the same as the position for the unpumped surface which is associated with the
generated vacuum space charge and will be different for the vacuum space charge with other
electrons’ kinetic energy cut-off. This case clearly demonstrates how vacuum space charge
affects the kinetic energy of Ga-3d photoelectrons. In the spectra shown in Fig. 6.7 the shift
due to the SPV effect is nearly the same but opposite to that of vacuum space charge. This
case also explains the difficulty to obtain the absolute SPV shifts in the presence of vacuum
space charge. The Ga-3d photopeaks show also the mentioned fast SPV transient shift
observed as a slight shift of the Ga-3d photopeak to lower kinetic energies after
photoexcitation (Fig. 6.7(c)).
For completeness Fig. 6.8 shows the measured SPV transient for the n-GaAs(100).
The phenomena yielding Ga-3d photopeak shifts in the opposite direction is the same as for
p-GaAs(100) only the roles of the electrons and holes are exchanged. After photoexcitation
the electrons in the SCR are drifting into the bulk and holes to the surface. This is leading to
the flattening of the electronic bands as well as core-levels originally bent upwards. The
Ga-3d photoelectron spectra for the positive time delay shorter than 1 ps show shifts to lower
kinetic energies. The surface recombination processes tend to restore the original Ga-3d peak
position for time delays longer than 1 ps.
76
(a)
(b)
Fig. 6.8 Ga-3d core-level peak as a function of the pump-probe delay - (a) contour plot and
(b) for selected time delays. The thin vertical lines in graph (b) indicate the peaks’
center-of-gravity positions.
77
6.2 Dynamics of the band-bending for p-GaAs(100)
An extraction of quantitative values for the SPV changes demands some attention
because of overlapping pump and probe beams with the same diameter of approximately
1.2 mm but an inhomogeneous spatial profile as shown simplified in Fig. 6.9. Photoelectrons
originating from the less pumped outer region therefore experience a correspondingly smaller
SPV shift as compared to the central region of the pump spot and the observed net shift
actually represents an integration over the whole area.
Ekin
Ga 3d
p-GaAs
Fig. 6.9 Principle of the asymmetrical Ga-3d photopeak broadening
Correspondingly, the measured Ga-3d peak profile s (E ) is given as an integral of the
unperturbed Ga-3d peak profile for the unpumped surface p (E ) and a weighting function
g (∆E ) describing the amplitude of the SPV effect as a function of the energy shift which in
turn depends on the spatial pump intensity profile I (x, y )
s( E ) =
∆Emax
∫ g (ς ) p( E − ς )dς
(6.1)
0
where ∆Emax is the maximum observed SPV shift. Deconvolution of the photoelectron peak
profile s (E ) requires exact knowledge of the spatial profile of the pump beam which has
been not recorded. Generally, there are three simple possibilities how to extract data on the
78
transient SPV shifts observed with my measurements. I restrict the analysis to the extraction
of the maximal SPV shift induced with the most intense part of the spatial pump pulse profile.
i) In a first approximation I can fit my data with two peaks, where one is fixed and the second
one is moving. Each of the peaks is a replica of the Ga-3d peak in the unpumped condition.
This method is quite good but it can not always fit properly the profile of the measured Ga-3d
peak. ii) Second possibility is to concentrate the attention only to the highest energetic edge of
the Ga-3d peak profile. In this case I can use a second derivative to find the inflexion point
which is tracking very precisely the maximum Ga-3d kinetic energy shift. However this
method is very sensitive to the peak profile and very good photoelectrons’ peak statistics is
required. iii) The last method is the center-of-gravity determination. This very robust method
has limited sensitivity for the subtle changes at the highest energy edge of the Ga-3d peak
because of the not weighting nature of the center-of-gravity calculation. Although reduced
sensitivity for the fine changes at the highest energy edge of the Ga-3d photopeak I use this
method as primer source of the information on the observed band-bending dynamical
processes. The center-of-gravity EC and its standard deviation (shown as error bars) ∆EC has
been calculated as follows269
EC =
1
∑ NE
∑E⋅N
E
(6.2)
E
E
∆EC =
1
∑ NE
∑ (E − E
C
)2 ⋅ N E
(6.3)
E
E
where N E denotes the number of photoelectrons with kinetic energy E and summation in E
runs over the region of interest. In the derivation of equation (6.3) the Poisson probability
distribution of the registered photoelectrons has been utilized.
In Fig. 6.10 the transient shift of the Ga-3d photopeak is shown as a function of the
time delay between pump and probe determined with the center-of-gravity calculation. The
reliability of the center-of-gravity shifts has been verified by repetitive measurement of the
shift for +1 ps time delay one hour later under the same experimental conditions. These
measured shifts are shown inside of the gray bar in Fig. 6.10(b). The calculated error bars in
Fig. 6.10 with equation (6.3) are showing the Gaussian standard error interval361 1σ
corresponding to 68.3% of all measured shifts.
(a)
(b)
Fig. 6.10 Temporal evolution of the Ga-3d peak’s center-of-gravity in linear (a) and
semi-logarithmic (b) time scale after photoexcitation of the p-GaAs(100) surface.
79
80
However the shift for +1 ps time delay measured one hour later does not fall in the standard
1σ confidence interval and only the confidence interval based on 2σ corresponding to 95%
of all measured shifts can justify the shift measured one hour later. A different value of the
observed shift points to possible deterioration of the surface conditions. To solve this problem
I have to achieve better vacuum of order 1x10-11 mbar. The pointing laser stability resulting in
the falling conversion efficiency for high harmonics (Fig. 3.9) and in the changes of the
spatial pump profile are also an error source contributing to the measured different values for
the SPV shifts. In order to reproduce the measured SPV shifts one needs the precisely same
prepared and reconstructed surface. Already small differences in the surface preparation are
yielding different SPV transients as shown in Appendix C. The temporal coincidence of pump
and probe pulses found with the interferometric method is precise within 100 fs due to small
required re-adjustments needed afterwards. Daily operation and measurements on highly
photoexcited n-GaAs(100) has proved a sudden shift of the Ga-3d peak at +100 fs. On this
basis I believe to find the pump-probe temporal coincidence in Fig. 6.10 at about +100 fs. In
Fig. 6.10 there are noticeable Ga-3d peak shifts for negative time delays. The reason for the
observed phenomenon could be found in the temporal pump pulse profile. The highly
dynamic autocorrelation measurements characterizing femtosecond pulses from CPA laser
systems365 show that the light intensity at 1 ps before the pulse arrival is some 10-6 of the
maximum intensity and at a few hundreds femtosecond some 10-3 of the maximum intensity.
Since the SPV effect is not linear136 with the pump intensity, at high pump fluences as used in
this study it tends to saturate. It is quite possible that already few hundreds femtoseconds
before the pump pulse arrival the intensities of some 10-3 of the pump pulse intensity are
contributing to the observed slight Ga-3d photopeak shifts. The Ga-3d shifts shown in
Fig. 6.10 are relative to the negative time delay at -800 fs. Fast carrier transport in the
photoexcited semiconductor bulk to the probed surface through an acceleration in the strong
SCR electric field manifests itself in a reduction of band-bending and a corresponding peak
shift within less then 500 fs after the pump pulse. The subsequent carrier relaxation by
recombination and trapping in surface states is observed to occur on a slower time scale148,151
of a few picoseconds. The dynamical behavior in Fig. 6.10 can be well fitted with a
convolution of an exponential decay and a gaussian function of the form
e
−
t
T
t
w 

1 + erf ( w − 2T )
(6.4)
81
where erf is the error function§. The fitting procedure based on the method of least squares is
yielding the mean values and their standard deviations of w = 600 ± 180 fs for the rise time
and T = 15 ± 3 ps for the decay time. Electrons transit times of about 500 fs in p-GaAs were
predicted by simple theoretical considerations139,145, assuming solely the electron drift with
the saturation velocity through the SCR, albeit for lower doping of 1018 cm-3 with a
correspondingly larger SCR. An extraction of more detailed information on the carrier
dynamics for the studied system will be obtained by solving the complex set of coupled
differential equations (2.19)-(2.21) connecting electron-hole creation, charge transport,
relaxation and modification of the electric field in the SCR by the transient charge
redistribution. For completeness the transient Ga-3d peak shifts for the n-GaAs(100) with
significantly lower dopant level is shown in Fig. 6.11. The observed Ga-3d peak shifts to
lower kinetic energies as has been expected. Surprisingly, the Ga-3d shift occurs within 400 fs
which is faster than for the p-GaAs surface. At this stage I need support by a theoretical
simulation to interpret the measured results.
Fig. 6.11 Temporal evolution of the Ga-3d peak’s center-of-gravity after photoexcitation of the
n-GaAs(100) surface.
§
Error function defined as erf ( x) =
2
x
e
π ∫
0
−t 2
dt
82
Chapter 7 – Conclusions and Outlook
Chapter 7
Conclusions and Outlook
In the framework of this thesis novel apparatus for the femtosecond time-resolved
photoelectron spectroscopy in the EUV region has been built. The CPA femtosecond laser
system based on regenerative and multipass amplification has been constructed and
characterised. By focusing the laser beam in the pulsed Ne gas jet the high harmonic has been
generated. The key position in the experimental apparatus is the low group velocity dispersion
EUV monochromator46 selecting single high harmonic without its temporal distortion. The
specially tailored multilayer low gamma Mo/Si mirrors47 has been applied for the selection
and refection of single high harmonic. Using the TOF electron spectroscopy and selected high
harmonic the first static photoelectron spectroscopy on the rare gases has been performed45.
The excellent spectral selectivity of the EUV monochromator has been investigated via
photoelectron spectroscopy of He gas. The UHV compatibility of this apparatus made the
photoelectron spectroscopy of the solid states surfaces also possible. The first photoelectron
and Auger electron spectra of the Pt(110) surface as well as Ga and As core-levels by the
photoemission from GaAs(100) surface has been observed. The chemical shift of the W 4f
core-levels has been studied on W(110) crystal surface and W(CO)6 adsorbate. The iodine
films 4d core-levels and valence band photoemission together with Auger electron spectra has
been observed. Also the Ga-3d core-levels and valence band photoelectron spectra of a
complex organic molecule Gaq3 has been investigated. The visible-pump/EUV-probe
technique has been implemented for the observation of the ultrafast hot electron relaxation on
Chapter 7 – Conclusions and Outlook
83
a Pt(110) surface48. The valence band electron dynamics of highly excited Pt surface has been
observed. The valence band depopulation effect due to the total excitation of 2.2% of all
valence band electrons has been examined. The following cooling of hot electrons
accompanied by the rise of crystal lattice temperate has been analysed. Due to short lifetime
of hot electrons of a few femtoseconds a visible-pump/EUV-probe cross-correlation has been
measured and constitutes the first UHV compatible technique for the measurement of
femtosecond high harmonic duration. The principle is based on the short living hot electrons
excited into the unoccupied states above the Fermi level and emitted with the time-delayed
high harmonic probe pulse. The visible-pump/EUV-probe time-resolved core-level
photoelectron spectroscopy has been utilized for tracking the charge carrier dynamics on
semiconductor surfaces. Transient changes of the surface photovoltage at the p-GaAs(100)
surface has been observed after photoexcitation with femtosecond pump pulses. The time
evolution of the band-bending has been probed by measuring the kinetic energy shifts of the
Ga-3d core-level photoelectrons after excitation with femtosecond EUV probe pulses. Carrier
transport from the bulk to the surface has been observed to occur within 500 fs after
photoexcitation while a subsequent relaxation has been determined to evolve on a time scale
of a few tens of picoseconds.
I hope that in this thesis I have convinced about usefulness, capability and great
potentials to address core-level electrons by the femtosecond time-resolved photoelectron
spectroscopy in the EUV region. An application of this experimental technique for the
observation of electron dynamics at high pump intensities on semiconductors surfaces366 and
the dynamics of chemical bonding of ordered organic layers on surfaces367 will give new
physical insights on these two promising branches of the ultrafast processes. It makes me
believe that my core-level based time-resolved experiment is predetermined to offer a deeper
view on highly photoexcited semiconductors surfaces not accessible with other time-resolved
techniques. The traditional time-resolved studies of the chemical bonds are based on the
changes induced in the bonding molecular orbitals. However, the complex organic molecules
consisting of large number of elements have the valence band structure which changes are not
easy to interpret and locate one particular site in the molecule where the chemical bond had
changed. The idea of “marker atom” embedded in the site of interest and application of
time-resolved photoelectron spectroscopy on the core-level electrons of this particular
“marker atom” will provide the information on the dynamics of complex molecular systems
in the very proximity of the “marker atom“ rather than on the “site-unspecific” changes of the
valence band.
84
Appendix
Appendix
A Knife-edge Beam Profile Measurement
The most simple technique of the EUV beam profile analysis is so-called knife-edge
technique. The EUV beam before the TOF electron spectrometer has been gradually blocked
by the knife-edge plate and the passing residual light has been measured with the MSP
intensified photocathode (see Fig. 3.1). The measured signal proportional to the EUV
intensity as a function of the knife-edge position is shown in Fig. A.1. On the assumption that
EUV beam has spatially homogenous Gaussian profile368 the measured data can be fitted with
a function
w
I w π
I ( x) = 0
2 2
2
x
∫e
−t 2
dt
(A.1)
−∞
where I 0 is the maximum of light intensity and 2w is the full width at e-2 of I 0 . Under these
conditions the fitted beamwidth is 2 w = 1.2 mm.
Fig. A.1 The measured EUV light intensity as a function of the knife-edge position. The grey
curve shows the corresponding fit on the assumption of the Gaussian beam profile.
Appendix
85
Appendix
B Auger Electron Spectroscopy on Pt, Cu and GaAs Surfaces
The Auger electron spectroscopy287 (AES) has been used to check on the quality of
surface cleanliness. The excitation electron beam with the kinetic energy of 1.25 keV has
been impinging under 75° to the surface normal. The AES spectra has been collected with
self-made retarding field analyser369.
Fig. B.1 The AES spectra of Pt(110) surface before (a) and after (b) cleaning procedure.
Thin vertical lines indicate the characteristic electron peaks of platinum and carbon.
The AES spectra of Pt(110) surface before and after cleaning are shown in Fig. B.1. The
initial carbon contamination has been removed during the cleaning procedure. In Fig. B.2 are
shown the AES spectra of Cu(100) surface before and after cleaning. In the region between
100 eV and 600 eV there are no characteristic lines of copper and only the main contaminants
like sulphur and carbon are visible. These were eliminated during the cleaning procedure. The
Fig. B.3 shows the AES spectra of GaAs(100) surface. Again in the measured region between
100 eV and 600 eV there are neither the characteristic lines of gallium nor arsenic. The carbon
86
Appendix
contaminants as well as native gallium and arsenic oxides were removed during the cleaning
procedure.
Fig. B.2 The AES spectra of Cu(100) before (a) and after (b) cleaning procedure. The
vertical lines show the position of electron peaks of carbon and sulphur contaminants.
Fig. B.3 The AES spectra of GaAs(100) before (a) and after (b) cleaning procedure. The
contamination due to carbon and native oxides show the vertical lines in the electron spectra.
Appendix
87
Appendix
C Surface Photovoltage Transients and Surface Preparation
The SPV transients are highly sensitive probe of the semiconductor surface conditions. This is
demonstrated in Fig. C.1-C.3. The p-GaAs(100) and n-GaAs(100) surfaces has been prepared
as described in section 4.1. Although the preparation conditions varied only in a few minutes
in the duration of the sputtering and annealing times the behaviour of the SPV transients were
radically influenced. This is shown for p-GaAs(100) in Fig. C.1-C.2 (compare with Fig. 6.10).
This has been also observed for n-GaAs(100) in Fig. C.3 (compare with Fig. 6.11).
Fig. C.1 Temporal evolution of the Ga-3d peak’s center-of-gravity after photoexcitation of
the p-GaAs(100) surface.
88
Appendix
the p-GaAs(100) surface.
the n-GaAs(100) surface.
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Eidesstattliche Erklärung
Hiermit versichere ich, die vorliegende Arbeit eigenständig und ohne fremde Hilfe
durchgeführt zu haben. Alle benutzten Hilfsmittel sind in der Literaturliste kenntlich gemacht.
Bielefeld, den 19.03.2002
Peter Šiffalovič
Curriculum Vitae
Personal
Name :
Date of birth :
Place of birth :
Nationality:
Martial status:
Peter Šiffalovič
23.05.1975
Bratislava
slovak
single
Education
1982 – 1989
1989 – 1993
1993 – 1998
July 1996
May – Nov. 1997
July 1998
since Sep. 1998
primary school in Bratislava
grammar school of J. Hronca in Bratislava
undergraduate study of physics – optics and optoelectronics at
Comenius University in Bratislava
practical training on femtosecond lasers at Technical University,
Vienna, Austria (prof. H. F. Kauffmann)
scientific sojourn at Shizuoka University, Research Institute of
Electronics, Hamamatsu, Japan - research on nonlinear properties of
organic crystals (prof. T. Nagamura)
master in physics with diploma work - Nonlinear phenomena in χ(3)
media
joined department of multilayers at Institute of Physics SAS, Bratislava
(prof. Š. Luby and Dr. E. Majková)
Ph.D. study at department of molecules and surfaces, University
Bielefeld, Germany (prof. U. Heinzmann)
Awards
1995, 1996, 1997
“Laureate of student scientific conference” for works on acousto-optics,
CCD-imaging and measuring of ultrashort laser pulses via TPA

Femtosecond Time-Resolved Photoelectron Spectroscopy in the

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