Upload
others
View
5
Download
0
Embed Size (px)
Citation preview
Martin WolfDepartment of Physics, Freie Universität Berlin
IMPRS Block Course “Chemistry and Physics of Surfaces”
Introduction to femtosecond laser spectroscopy and ultrafast dynamics at surfaces
structure
kinetics
dynamics
E. Muybrigde 1887
Principle: Stroboscopic investigation of motion and structural changes
Application of femtosecond laser spectroscopy
Goal: Microscopic understanding of elementary processes at surfaces
Outline
IntroductionWhy femtosecond laser pulses?
Example: photochemistry of vision
Generation of femtosecond laser pulses
Laser basics and mode locking
hot e-
E
EF
e-transfer
substrate adsorbate
HOMO
LUMO
Femtochemistry
Vibrational wave packet dynamics
Surface femtochemistry
Electron and phonon mediated pathways
SFGNon-linear optics as a probe of surface dynamics
Vibrational sum-frequency generation spectroscopy
Why femtosecond spectroscopy ?Time scales of chemical reactions:
Temporal evolution from reactants to products:
Dynamics of the transition state
Typical timescale: 10-100 fs
Need femtosecond laser pulses for direct observation in time domain
Rhodopsin
Eye
Example: photochemistry of vision
Rod cells: A single rod contains 108 rhodopsins(renewed every 10 daysthroughout a person’s life)
cis-trans isomerization
Retinal
hν
11-cis all-trans
How do we see ?
Hirarchy of time scales
Optical absorption changes during reaction cycle
ultrafastconformationalchanges
therrmal relaxationde-protonation
Study time-resolved changes of optical absorption spectrum
pump
probe
signal
∆t
Time resolution is determined by pulse duration
Pump-probe spectroscopy
Basic concept to resolve processes on ultrafast time-scales
reflection transmission
Primary step of vision
How can this process be so fast ?
Spectral changes after excitation
Primary step (cis-trans isomerization) of Retinal must occur within 200 fs !
pump@ 500 nm
∆α
t ~ 0
t = 200 fs
“Vibrational coherent wave packet motion”
excited stateabsorption
(t ~ 0)
probe
History of time resolution
Year
time
reso
lutio
n(s
)
A.H. Zewail
Nobel prize 1999
Break through by development of pulsed laser sources
Development of short laser pulsespu
lse
dura
tion
(s)
„mode locking“
„Titanium Sapphire“
Year
normal dispersion(e.g. glass)
Properties of ultrashort laser pulsesShort pulse: Electrical field E(t) = ε(t) cos(ω0t + ϕ) with envelope ε(t)
time bandwidth product
(transform limit)
ε(t) ε(ω)
Generation of fs-laser pulses
A generic ultrafast laser consits of a broadband gain medium,a pulse-shortening device, and a resonator:
Why ?
Laser basics ILASER: Stimulated emission
Example: Four level laser (Nd:YAG)
Populationinversion
Titanium-sapphire laser
Laser basics II Resonator modes:
longitudinal modes:
Stability:g2 = 1-d/R2
g1 = 1-d/R1
R1 R2d
g1 g2 = 1
0 ≤ g1 g2 ≤ 1
condition for a stable resonator:
refractive index
Fabry-Perot
Mode-locking
Synchronization of laser modes:One mode:
All modes:
Constant phase between modes
Frequency domain:
Temporal structure:
Random phases
∆t = 1/∆ν
m
Mode-locking
„2 beam interference“
non-linear optical Kerr effectInduced polarization in non-linear optical medium
Linear optics: n0 = √ε = √ε0(1+4πΧ(1))
Non-linear optics:
Longitudinal and transversal Kerr effect
“Kerr lens”
1) self phase modulation
2) self focussing“Kerr lens”
Kerr lens mode-locking and dispersion
Pulse broadening due to dispersion
Example: Ti:Sa laser
Prism compressor (GVD compensation)
Measurement of pulse duration
auto correlation
interferrometric autocorrelation
I(τ) =
Outline
IntroductionWhy femtosecond laser pulses?
Example: photochemistry of vision
Generation of femtosecond laser pulses
Laser basics and mode locking
Surface femtochemistry
Electron and phonon mediated pathways
Non-linear optics as a probe of surface dynamics
Vibrational sum-frequency generation spectroscopy
Femtochemistry
Vibrational wave packet dynamics
hot e-
E
EF
e-transfer
substrate adsorbate
HOMO
LUMO
SFG
Real-time probing of chemical reactions
Real-time probing of chemical reactions II
Example: photodissociation
Analysis of repulsive excited state PES
“Clocking of chemical reactions”
Real-time probing of chemical reactions III
“Coherent wave packet motion”
reaction product
transition state
Outline
IntroductionWhy femtosecond laser pulses?
Example: photochemistry of vision
Generation of femtosecond laser pulses
Laser basics and mode locking
hot e-
E
EF
e-transfer
substrate adsorbate
HOMO
LUMO
Femtochemistry
Vibrational wave packet dynamics
Surface femtochemistry
Electron and phonon mediated pathways
SFGNon-linear optics as a probe of surface dynamics
Vibrational sum-frequency generation spectroscopy
Laser induced surface dynamics
Question: How does laser excitation induce surface reactions ?
Electronic excitations and charge transfer
Chemical bonding implies electronic couplingbetween „adsorbate“ and substrate
Vibrational relaxation at metalsby electron-hole pair excitation
hot e-
E
EF
e-transfer
substrate adsorbate
HOMO
LUMOElectron stimulated desorptionand surface photochemistry
Energy transfer by coupling betweenelectronic and nuclear degrees of freedom
Thermal activation
Phonon driven process (in electronic ground state)
kBT
2 CO + O → CO ↑adsads
fs-laser-induced chemistry at surfaces
Example: CO oxidation versus desorption on Ru(001) single crystal surface
hν2 CO + O → CO ↑ + CO2↑adsads
Co-adsorption of O+CO on Ru(001)(UHV conditions)
Flux
First ShotYield
200150100500Tim e-o f-F ligh t (µs)
1590 KEtrans =
640 K
Flux
[arb
. uni
ts]
fs-induced reaction pathways
CO Oxidation
CO desorption
Etrans =
+ kT
New reaction pathway is „switched on“ upon fs excitation
metal
Optical excitation of metal surfaces
Mechanism and timescales of energy transfer after optical excitation
~0.1-1 ps
~1 pselectrons
Tel
direct absorptionfs-excitation reaction
>1 ps
~100 fs
phononsTphm
etal
adsorbate vibrations Tads
Photoabsorption in a metal substrate creates a transientnon-equilibrium electron distribution
Energy transfer to phonons and adsorbate vibrational excitation
Primary step: Electron thermalization and cooling
Electron thermalization dynamics in metals probed with 2PPE and the two-temperature model
Electron dynamics in metals following optical excitation
The two-temperature model
Nearly thermal (Fermi-Dirac)distribution for electrons
Model assumes two heat baths for electrons and phonons: Cel >> Cph
PdCoupled diffusion equationfor Tel and Tph
Time-resolved two-photon photoemission How to probe electron thermalization and cooling ?
2kinF hEE-E ν−Φ+=
EF
Evac
Ekin
hν1
hν2
hν
e-
Φ
ϕ
/mE2sink kin|| hϕ=
TOF
e-
energy- and time-resolution:
pulseduration:65 fs
momentum resolution:
e-UV probe pulse
pump pulse
e-e scattering
Electron thermalization dynamics:
hot electroncascades
Experimental setup
Femtosecond time-resolved two-photon photoemission spectroscopy
µJ
W. S. Fann, R. Storz, H.W.K. Tom, J. Bokor,Electron thermalization in gold, Phys. Rev. B 46, 13592 (1992)
Method: time-resolved photoemissionwith hνprobe > Φ
Pump fluence: 120 µJ/cm2
300 µJ/cm2
Electron thermalization in gold
Electron thermailization occurs fasterwith increasing excitation density
Fermi-Dirac distribution
Time-resolved 2PPE from Ru(001)
0.0001
0.001
0.01
0.1
1
2PP
E in
tens
ity
1.51.00.50.0
E - EF (eV)
0.0001
0.001
0.01
0.1
1
0.0001
0.001
0.01
0.1
1
580 µJ/cm2
230 µJ/cm2
40 µJ/cm2∆ t / fs
0100200300400500
Ru(001)
e-UV probe800 nm pump
Pump-probe scheme:
probing electron dynamics around EFv
e-e scattering
1) Electron thermalization:
2) Relaxation by e-ph scattering
3) Energy transport in the bulk
Lisowski et al., Appl. Phys. A 78, 165 (2004)
2
4
68
10-6
2
4
68
10-5
2
4
68
elec
tron
ener
gy[J
/cm2 ]
9006003000-300time delay (fs)
Extended heat bath model
efficient ballistic transportof non-thermal electrons
Goal: Understanding the role of non-thermalized electrons
however, peak energy is describedreasonable well by both models
2TM underestimates rate of energy flow into the bulk
Extended modelincluding ballistic transport
2TM
Extension of two temperature modelAppl. Phys. A 78, 165 (2004)
transientexzess energy
of electrons
2TM
Femtochemistry at metal surfaces
substrate-mediated excitation mechanism dominates
Anisimov, et al. Sov. Phys. JETP , 375 (1974)39
120 fs50 mJ/cm2
phonon
electron
Time (ps)
Tem
pera
ture
(K)
T
T
1 20
Mechanism and timescales of energy transfer after optical excitation
~0.1-1 ps
~1 pselectrons
Tel
direct absorptionfs-excitation reaction
>1 ps
~100 fs
phononsTphm
etal
adsorbate vibrations Tads
transient non-equilibrium between electrons and phonons: Tel >> Tph
new reaction mechanism?by non-thermal activationby separation of time scales of energy flow
Reaction mechanisms
Phonon-mediated Electron-mediated
Desorption Inducded byMultiple Electronic Transitions
DIMET -
substrate
adsorbateE
reaction coordinate
phonons Edes
hot e-
E
EF
τ ~1-10 fs
ground state
stateexcited
Edes
e-transfer
reaction coordinatesubstrate
kBTph
How to distinguish?
DIMET mechanismDesorption induced by multiple electronic transistions
NO/Pd(111)
see also Brandbyge et al., PRB 52, 6042 (1995)
Experimental Investigations
• NO and O2/Pd• CO/Cu • CO/Pt• O2/Pt • CO/NiO• CO + O2/Pt• CO + O/Ru, H+H/Ru
Misewich, Loy, HeinzTom, PrybylaHo MazurStephenson, Richter, CavanaghBonn, Wolf, ErtlZacharias, Al-ShameryDomen
YieldFluence dependenceTranslational energyVibrational energy Rotational energyUltrafast dynamicsInfluence of laser pulse durationInfluence of photon energyDependence on adsorption siteDependence on coverageCompetitive reaction pathwaysIsotope effects
Surface femtochemistry
Systems
2 CO + O → CO ↑adsads
Example I: Desorption and oxidation of CO on Ru(001)
Oxidation of CO impossible under equlilibrium conditions
200150100500Time-of-Flight (µs)
CO: 640 K
CO2: 1590 KEtrans =
4003002001000
Absorbed Fluence (yield weighted ) [J/m2]
Y(CO2) ~ F3.2
Y(CO) ~ F3.1
x 10F
lux
Firs
t Sho
t Yie
ld
hν2 CO + O → CO ↑ + CO2↑adsads
(1x2) - O/Ru(001)+ CO saturation
New reaction pathway upon fs excitation
Example II: Recombinative desorption of H2 on Ru(001)
H2 formation induced by fs laser excitation of H/Ru(001)
Full monolayerof H/Ru(001)
QMS
H2
H
Ru(001)
H
= 10 ± 2.4Yield (H2)Yield (D2)
H2: (2110 ± 400) K
flight time [µs]
flux[a.u.]
60
(x 10)
40200
0
D : (1720 ± 290) K2
<E >/ 2kkin B
High translational temperature of desorbing species
Pronounced isotope effects in H2/D2 yield
Temperature profiles and 2-pulse correlation
Tph
Tel6000
5000
4000
3000
2000
1000
020151050
time [ps]
surfa
cete
mpe
ratu
re[K
]
pulse 1 pulse 2
FWHM ~ 10 ps
FWHM ~ 1 ps
pulse-pulse delay [ps]re
actio
nyi
eld
[a.u
.]
-200 -100 0 100 200
phonon-mediated: slow response
electron-mediated:fast response
Goal: Discerning electron and phonon driven mechanismsQMS
CO
Ru(001)
CO
O
CO2delay
Two-pulse correlation measurement of the reaction yield allowsto distinguish between the two reaction mechanisms!
Experimental setup
Ti:sapphire oscillator + amplifier 4 mJ 800 nm, 110 fs, 20-400 Hz,
2 equally intensepulses
Ru(001)
QMS
H2/D2
CCD
pulse profilediagnostics
/
UHV
“Feulner cup“ detector
80 mm
O = 33 mm/
Ru(001)
QMSionisationvolume
H2
CO
2-pulse correlation measurements
120 fs50 mJ/cm2
phonon
electron
Time (ps)
Tem
pera
ture
(K)
T
T
1 20
CO
O
∆t
Ru
Tel Tph
CO
CO2
800 nm 120 fs
-200 -100 0 100 200
x24 CO2
COfir
stsh
otyie
ld(a
.u.)
pulse-pulse delay (ps)
FWHM=3 ps
FWHM=20 ps
2-pulse correlation:
Firs
t Sho
t Yie
ld
Goal: discriminate between electron and phononmediated mechanism
: Oxidation hot electron-mediated
: Desorption phonon-mediated
DFT calculations C. Stampfl, M. Scheffler, FHI Berlin
unoccupiedanti-bonding
orbital ~1.7eVabove EF
increasing Tel leads to
weakening ofRu-O-Bond
Ru(001)
O
Hot electron-induced activationof the O-Ru bond
Mechanism for CO oxidation
hotelectrons
EFermi
E O-level
anti-bonding
reaction coordinate
electronshot
Ru(001)
E
CO + O
0.3 eV
4.7 eV
1.7eV
from DFT
V = 1V = 2
O + CO
CO
CO + O
ad ad
ad
2,g
g g
gad
E ~1.8 eVa
Hot electron mediated vibrational excitation of the O-Ru bond
Non-adiabatic energy transfer from electronic excitations to adsorbate coordinate
M. Bonn et al, Science , 1042 (1999)285
Discussion
Reaction pathways for thermal versus femtosecond excitation
E
reaction coordinate
O
O
C
O*thermal excitation
O
0.8 eV
CO
O
O
CO
O
CO*
O
C
fs-laser excitation
1.8 eV
1.0 eV1.8 eV
e-
kT
Under thermal excitation the CO has desorbed before oxygen is activated
Seperation of time scales opens new reaction pathway
Electronic friction model
Energy transfer by coupling betweenadsorbate vibration and metal electrons
Electronic friction:
hotelectrons
EFermi
E
leveladsorbate
electronshot
metal
Coupling rate η = ~ ⎯ ⎯ 11Melτ
Coupling rates: τel τph τel-ph-1 -1 -1
., ,
met
al
τph
τel-ph Tph
τel
elTheat diffusion
adsT
ads-E /k Ta bRate ~ e
Two-Temperature Model
Coupled heat baths: el ph ads T , T , T What is “electronic friction”?Example: Change of electric resistivityin a thin metal film upon Xe adsorption
-V Xe
~1 ML
Struck PRL , 4576 (1996) ; Brandbyge .PRB , 6042 (1995).et al. et al77 52
2PC of laser-induced H2 formation
Coupling rates: τel τph τel-ph-1 -1 -1
., ,
met
al
τph
τel-ph Tph
τel
elTheat diffusion
adsT
ads-E /k Ta bRate ~ e
Two-Temperature Model
Coupled heat baths: el ph ads T , T , T Ultrafast response indicatescoupling to hot electron transient
first
shot
yiel
d[a
.u.]
pulse-pulse delay [ps]
H2exp. data
FWHM = 1.1 ps
0
-10 -5 0 5 10
model
Brandbyge et al., PRB 52, 6042 (1995)
Electronic friction model yields: Ea = 1.35 eV and τel = 180 fs(τel = 360 fs for D2)
for H2
Isotope effect in recombinative desorption
H2 formation induced by fs laser excitation of H/Ru(001)
Full monolayerof H/Ru(001)
QMS
H2
H
Ru(001)
H
= 10 ± 2.4Yield (H2)Yield (D2)
H2: (2110 ± 400) K
flight time [µs]
flux[a.u.]
60
(x 10)
40200
0
D : (1720 ± 290) K2
<E >/ 2kkin B
Isotope effect
Pronounced isotope effects in H2/D2 yield
Isotope effect and electronic friction model
Energy transfer by coupling betweenadsorbate vibration and metal electrons
Electronic friction:
hotelectrons
EFermi
E
leveladsorbate
electronshot
metal
Coupling rate η = ~ ⎯ ⎯ 11Melτ
Brandbyge et al., PRB 52, 6042 (1995)
pote
ntia
l ene
rgy
Ru-H/D distance
r electronicallyexcited PES
ground state of substrate-adsorbate-complex
Origin:- mass-dependent distance
traversed on excited PES
- lighter adsorbate starts movingmore rapidly
Fluence dependence: isotope ratio and first shot yield
0
160140120100806040200
0
403020100laser shot #
<F>~75J/m2 H counts [a.u.]
2
first
shot
yiel
d[a
.u.]
isotoperatio
20
10
2001601208040
experimental data
5
15
25
0
isot
ope
ratio
Y(H
2)/Y
(D2)
electronic friction model
Electronic friction model* simultanouslydescribes: • 2-pulse correlation
• fluence depedence of yield and isotope ratio
* Brandbyge et al., PRB 52, 6042 (1995)
exp. data model power law fit
Y(H2) ~ <F>2.8 and Y(D2) ~ <F>3.2
Outline
IntroductionWhy femtosecond laser pulses?
Example: photochemistry of vision
Generation of femtosecond laser pulses
Laser basics and mode locking
hot e-
E
EF
e-transfer
substrate adsorbate
HOMO
LUMO
Femtochemistry
Vibrational wave packet dynamics
Surface femtochemistry
Electron and phonon mediated pathways
SFGSFGNon-linear optics as a probe of surface dynamics
Vibrational sum-frequency generation spectroscopy
Real-time probing of vibrational dynamics
Goal: Probe suface reactions directly in the time domain
~0.1-1 ps
~1 pselectrons
Tel
direct absorptionfs-excitation reaction
>1 ps
~100 fs
phononsTphm
etal
adsorbate vibrations Tads
So far probed only the (desorbing) reaction products
Use surface sensitive non-linear optics (SHG, SFG) to obtain a direct look inside the excited adlayer
sum-frequency generation (SFG)
Background-free, visible detectionSensitivity: 10 MLShort pulses: time resolutionSurface specific
-3
Pot
.Ene
rgy
C-O distance
v=0
v=1
v=2
5 µm + 800 nm = 690 nm
IR
SFGVIS
With background, IR detectionoptical linear technique
C-O distance
IRv=0
v=1
v=2
Pot
.Ene
rgy
IR absorption spectroscopy(FTIR)
Vibrational spectroscopy
CO
C-O stretch
Vibrational SFG spectroscopy
Principle of sum-frequency generation (SFG):
Pot
. Ene
rgy
C-O distance
IRv=0
v=1
v=2
SFGVIS
CO
C-O stretch
P E E = χ ω(2) (2)
IR ωVISω ωSFGSFG
ωSFG = ωIP + ωVIS
Surface sensitive method:SFG is symmetry forbidden in isotropic (bulk) media (dipole approximation)
Second order non-linearoptical process
Time-resolved SFG as molecule specific probe of surface reactions
Broadband IR vibrational SFG spectroscopy Vibrational spectroscopy without tuning the IR frequency
IR
VIS SFG
IR
VIS
RuSFG
IRIR
VIS
VIS
Γn
ΓSFG
01
2
~5 cm-1
~150 cm-1
SFG spectrum ( 3x 3)-CO/Ru(001) √ √
SFG wavelength [nm]690 688 686 684694 692
SFG
inte
nsity
IR
20001950
FWHM9 cm-1
1900 2050IR wavenumber [cm ]-1
2100 2150
200 K
SFG
IR intensity
Use spectrally broad pulse with spectrally narrow upconversion pulse
fs- IR VIS
L.J. Richter et al., Opt. Lett. 23, 1594 (1998).
SFG experimental setup
Ti:sapphire oscillator + amplifier4 mJ, 800 nm , 110 fs, 20-400 Hz,
OPG/OPA (TOPAS), 20-30 J µ2.5-10 mµ
tunableIR pulse
pump pulse
spectrometerCCD camera
multi-channel detectionRu(001)
QMS
SFGprobe
pulse shaperFWHM 7 cm , 7 J µ-1
narrow bandVIS pulse
Example water/Ru(001)
intrinsically surface sensitive
SFG signal is enhancedwhen IR Photon is resonantwith vibrational transition
=
Sum-frequency generation (SFG)
Example: D2O covered and bare Ru(001) surface
D2O multilayers: free OD resonance
bare Ru: nonresonant background
VISSFGIR
VIS field
543210time (ps)
IR fieldIR polarization
2) Frequency-resolved SFG: temporally long, spectrally narrow VIS-pulse
1) Time-resolved SFG: temporally short, spectrally broad IR + VIS-pulse
SFG
inte
nsity
420time (ps)
240023002200IR frequency (cm-1)
SFG spectrum
SFG
inte
nsity
I~e-2t/T2
Γ~1/T2
Equivalence of spectral- and time-domain measurements
FID
linewidth
VIS IR SFG
Au
VIS
delay
IRSFG
Au
Experimental time resolution: better than 150 fsExperimental frequency resolution: better than 3 cm -1
CH3CN CH3CN
80
70
60
50
40
30
x103
25002400230022002100 IR frequency (cm-1)
SFG
inte
nsity
(arb
. uni
ts) C-N stretch
IR spectrum
SFG spectrum
Frequency domain: Time domain:
IR-broadband SFG: Spectral and time-resolution
M.Bonn et al. (Univ. Leiden)
80
70
60
50
40
30
x10 3
SFG
inte
nsity
(arb
. uni
ts)
43210
IR-VIS delay (ps)
C-N stretch
non-resonant responsefrom Au surface
Γ=7.6 cm-1 homogeneous→ T2=0.68 ps
SFG
inte
nsity
(arb
. uni
ts)
320031003000290028002700
IR Frequency (cm-1)
C-H stretchΓ=8.0 cm-1
→ T2=0.64 ps
SFG
inte
nsity
(arb
. uni
ts)
210IR-VIS delay (ps)
C-H stretch
T2=0.61 ps
C-N stretch
SFG
inte
nsity
(arb
. uni
ts)
43210
T2=0.68 pshom.
∆ω=2.5 cm-1; T2=1.65 psinhomogeneous
Probing surface inhomogeneities with SFG
25002400230022002100
SFG
inte
nsity
(arb
. uni
ts)
C-N stretch
∆ω=2.5 cm-1 inhomogeneous T2=1.65 ps
Time-domain measurements are particularly sensitive to inhomogeneitiesin surface adsorption sites due to temporal separation of interferences.
Time-resolved SFG of CO/Ru during desorptionSpectroscopic snapshots of the CO stretch mode
SFG
inte
nsity
(a.u
.)
21002050200019501900IR wavenumber (cm-1)
3 ps
23 ps
0.5 ps×8
×8
×8
Pump-Probe delay –4.5 ps–2.0 ps–1.5 ps–1.0 ps–0.5 ps0.5 ps3.0 ps
23.0 ps68.0 ps
168.0 ps
CO
yie
ld (a
.u.)
100shot number
210020001900
T =340 K, F=55 J/m02
CO/Ru(001)opticalprobe
Pronounced transient red shift, linewidthbroadening and decrease of intensity
Problems:Dipole-dipole coupling in the adlayerand small concentration of „products“
M. Bonn Phys. Rev. Lett , (2000) 4653et al, 84
Time-resolved SFG of CO/Ru(0001)
Calculation:
- anharmonic couplingto CO stretch mode
- thermal excitation offrustrated translation
At high fluence:
- excitation and couplingto higher frequency modes
Strong redshift indicated thatfrustrated rotation is dominante
Low fluence: Good fit
Real-time probing of surface reactions: 2PPE Snapshots of excited electronic states during evoltution of wavepacket
Time-resolved 2PPE spectra ‚Desorption‘ of Cs/Cu(111)
photoemissionprobe
Summary
IntroductionWhy femtosecond laser pulses?
Femtochemistry
Vibrational wave packet dynamics
Laser basics and mode locking
Generation of femtosecond laser pulses
Electron thermalization in metals
Test of the two-temperature model
Surface femtochemistry
Electron and phonon mediated pathways
Isotope effects and electronic friction model
Vibrational sum-frequency generation spectroscopy
SFG
0.0001
0.001
0.01
0.1
1
2PP
E in
tens
ity
1.51.00.50.0
E - EF (eV)
∆t / fs 0 100 200 300 400 500
2PP
E In
tens
ity
Ru
thanks to
C. Gahl, U. Bovensiepen, J. Stähler, M. Lisowski, P. Loukakos,
D. Denzler, S. Funk, C. Hess, and G. Ertl, Fritz-Haber-Institut Berlin
S.Roke, M. Bonn, Leiden University, The Netherlands
S. Wagner, C. Frischkorn, Freie Universität Berlin
Literature:H. L. Dai and W. Ho, Laser Spectroscopy and Photochemistry at Metal SurfacesWorld Scientific (1995).
H. Petek and S. Ogawa, Femtosecond time-resolved two-photon photoemission studiesof electron dynamics in metalsProgress in Surface Science 56, 239-311 (1998).
H. NienhausElectronic excitations by chemical reactions at metal surfacesSurface Science Reports 45, 1-78 (2002).
P.M. Echenique, R. Berndt, E.V. Chulkov, Th. Fauster and U. Höfer, Decay of electronic excitations at metal surfacesSurface Science Reports 52, 219-318 (2004).
M. Bonn, H. Ueba and M. WolfTheory of sum-frequency generation spectroscopy of adsorbed moleculesby density matrix method - Broadband vibrational SFG and applicationsJ. Phys.: Condens. Matter 17, 201 (2005)