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structure
kinetics
dynamics
E. Muybrigde 1887
Martin WolfDepartment of Physics, Freie Universität Berlin
Application of femtosecond laser spectroscopy
“Modern Methods in Heterogeneous Catalysis Research“ - WS 04/05
Time-resolved Studies of Surface Reactions
Goal: Microscopic understanding of coupling between electronic excitations and nuclear degrees of freedom
Timescales of chemical reactions
Typical timescale: 10-100 fs
Temporal evolution from reactants to products:
Dynamics of the transition state
Key concept: Dynamics on Born-Oppenheimer potential energy surface
Non-adiabatic coupling between electronic states near (avoided) crossings
QMS
Ru(0001) T = 95 KCO
Ru(0001)
O
T=95K
CO
800 nm, 120 fs~ 50 mJ/cm2
CO2
2 CO + O → CO ↑adsads
Motivation: fs-laser-induced chemistry
Example: CO Oxidation on Ru(001) competes with CO desorption (UHV conditions)
New reaction pathway is „switched on“ upon fs excitation
hν2 CO + O → CO ↑ + CO2↑adsads
(1x2) - O/Ru(001)+ CO saturation
Flux
First ShotYield
200150100500Tim e -o f-F lig h t (µs )
CO2 : 1590 K
Etrans =
CO : 640 K
Flux
[arb
. uni
ts]
fs-induced reaction pathways
Outline
IntroductionNon-adiabatic processes at surfaces
Surface femtochemistry
Electron and phonon mediated pathways
Isotope effects and electronic friction model
Electron thermalization in metals
Test of the two-temperature model
metal semiconductor
e-h pair
e-
Non-linear optics as a probe of surface dynamics
Vibrational sum-frequency generation spectroscopy
log
(2PP
E In
tens
ity)
0.60.30.0-0.3
E-EF (eV)
Ru(001)
non-thermalizedelectrons
Electron dynamics thin ice layers on metals Ru(001)
SFG
Gas surface interaction
Role of non-adiabatic processes in surface reactions ?
Example: Adsorption at a metal surface
E
R
„forced-oscillator model“
Energy dissipation via phonon excitation
Coupling to electron-hole pairexcitations in the substrate
Exothermic reactions at metal surfaces
Question: Coupling between nuclear(vibrational) motion of adsorbate and electronic excitations ?
Chemicurrents
See review article by H. Nienhaus, Surf. Sci. Rep. 45, 3 (2002)
Evidence for non-adiabaticcoupling between adsorbatemotion electronic excitations
Direct observation of e-h pair excitation
Adsorption on thin metal film
Charge separation acrosssmall Schottky barrier
Chemicurrent
Current provides lower limitof excitation probability
Chemicurrents: Mechanism
Newns-Anderson model
Chemiluminescence Exoelectron emission
Unoccupied affinity level ispulled down by adsorbatesurface interaction
Filling of hole below EFermiby substrate electrons
broadening of resonance linewidth
adsorbate substratecharge transfer
e-h excitation:- chemicurrent- exoemission
luminescence
Electronic excitations at surfaces
metal semiconductor
e-h pair
e-
Gergen et.al., Science 294, 2521 (2001)
Chemicurrent observed forvarious adsorbates
chemical reactions drivenby electronic excitations
Energy dissipation via e-h pair excitation („electronic friction“)
Feasibility of reverse process?
electronic excitations playmaior role in gas surfaceinteraction
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
metal
Optical excitation of metal surfaces
Relaxation to phonon and adsorbate vibrational excitation
Mechanism and timescales of energy transfer after optical excitation
Photoabsorption in a metal substrate creates a transientnon-equilibrium electron distribution
Primary step: Electron thermalization and cooling
~0.1-1 ps
~1 pselectrons
Tel
direct absorptionfs-excitation reaction
>1 ps
~100 fs
phononsTphm
etal
adsorbate vibrations Tads
Electron thermalization dynamics in metals: Test of the two-temperature model using 2PPE
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
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:
How to probe electron thermalization and cooling ?
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
Lisowski et al., Appl. Phys. A 78, 165 (2004)
e-e scattering
1) Electron thermalization:
2) Relaxation by e-ph scattering
3) Energy transport in the bulk
Prediction from 2TM
Analysis of electron distribution
Comparison with 2TM:
Assume thermalized distribution
for electrons and phonons
Thermalized electrons carry onlyminor fraction of excess energy
Non-eqilibrium carrier distribution:
non-thermalized ‚hot‘ electrons
thermalized Fermi distribution
measured Tel
2TM over-estimates final Tel
DOS of Ru and Cu
-0.05
0.00
0.05
norm
aliz
ed p
ump-
indu
ced
varia
tion
0.60.30.0-0.3E-EF(eV)
delay / fs0100200300500
Dynamics of electrons and holes
e-probe
Pump induced changes of electron distribution:
Highly symmetric electron and hole distributions for all delaysdue to nearly constant DOS around EF
Dynamics of electrons and holes can be treated equivalently
Excess energy of electrons + holes
electrons
holes
pump
Ultrafast relaxation (much faster compared to noble metals, e.g Au: 1-2 ps)
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
2
4
68
10-6
2
4
68
10-5
2
4
68
elec
tron
ener
gy[J
/cm2
]
9006003000-300time delay (fs)
Extended modelincluding ballistic transport
2TM
2TM
Extension of two temperature modelAppl. Phys. A 78, 165 (2004)
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
per
atu
re (
K)
T
T
1 20
Mechanism and timescales of energy transfer after optical excitation
transient non-equilibrium between electrons and phonons: Tel >> Tph
~0.1-1 ps
~1 pselectrons
Tel
direct absorptionfs-excitation reaction
>1 ps
~100 fs
phononsTphm
etal
adsorbate vibrations Tads
new reaction mechanism?by non-thermal activationby separation of time scales of energy flow
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 pathways
Surface femtochemistry
Systems
QMS
Ru(0001) T = 95 KCO
Ru(0001)
O
T=95K
CO
800 nm, 120 fs~ 50 mJ/cm2
CO2
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
New reaction pathway upon fs excitation
hν2 CO + O → CO ↑ + CO2↑adsads
(1x2) - O/Ru(001)+ CO saturation
Example II: Recombinative desorption of H2 on Ru(001)
H2 formation induced by fs laser excitation of H/Ru(001)
Pronounced isotope effects in H2/D2 yield
High translational temperature of desorbing species
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
/ 2kkin B
Reaction mechanisms
Electron-mediatedPhonon-mediated
How to distinguish?
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
Temperature profiles and 2-pulse correlation
TphTel
6000
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 mechanisms
Two-pulse correlation measurement of the reaction yield allowsto distinguish between the two reaction mechanisms!
QMS
CO
Ru(001)
CO
O
CO2delay
Experimental setup
Ti:sapphire oscillator + amplifier 4 mJ 800 nm, 110 fs, 20-400 Hz,
2 equally intensepulses
Ru(001)
QMS
H2/D2CCD
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
per
atu
re (
K)
T
T
1 20
CO
O
∆t
Ru
Tel Tph
CO
CO2
800 nm120 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
M. Bonn et al, Science , 1042 (1999)285
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
Discussion
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
Reaction pathways for thermal versus femtosecond excitation
Seperation of time scales opens new reaction pathway
2PC of laser-induced H2 formation
Ultrafast response indicatescoupling to hot electron transient Coupling rates: τel τph τel-ph-1 -1 -1 ., ,
met
al
τphτel-ph Tph
τel
elTheat diffusion
adsT
ads-E /k Ta bRate ~ e
Two-Temperature Model
Coupled heat baths: el ph adsT , T , T
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)
Pronounced isotope effects in H2/D2 yield
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
/ 2kkin B
Isotope effect
Energy transfer by coupling betweenadsorbate vibration and metal electrons
Electronic friction:
hotelectrons
EFermi
E
leveladsorbate
electronshot
metal
Coupling rate η = ~ 11 Melτ
Isotope effect and electronic friction model
Brandbyge et al., PRB 52, 6042 (1995)
pote
ntia
lene
rgy
Ru-H/D distance
relectronicallyexcited 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
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) ~ 2.8 and Y(D2) ~ 3.2
0
160140120100806040200
0
403020100laser shot #
~75J/m2 Hcounts
[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
~0.1-1 ps
~1 pselectrons
Tel
direct absorptionfs-excitation reaction
>1 ps
~100 fs
phononsTphm
etal
adsorbate vibrations Tads
Real-time probing of vibrational dynamics
So far probed only the (desorbing) reaction products
Goal: Probe suface reactions directly in the time domain
Use surface sensitive non-linear optics (SHG, SFG) to obtain a direct look inside the excited adlayer
Vibrational spectroscopy: Sum-frequency-generation
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 (in dipole approximation)
Second order non-linearoptical process
Time-resolved SFG as molecule specific probe of surface reactions
Broadband IR vibrational SFG spectroscopy
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
IR
VIS SFG
IR
VIS
Ru
Vibrational spectroscopy without tuning the IR frequency
SFG
IRIR
VIS
VIS
Γn
ΓSFG
01
2
~5 cm-1
~150 cm-1
Use spectrally broad pulse with spectrally narrow upconversion pulse
fs- IRVIS
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
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
yiel
d(a
.u.)
100shot number
210020001900
T =340 K, F=55 J/m02
CO/Ru(001)
Time-resolved SFG of CO/Ru during desorption
IR
fs pump
CO
Ru(001)
VIS SFG
M. Bonn Phys. Rev. Lett , (2000) 4653et al, 84
Spectroscopic snapshots of the CO stretch mode
opticalprobe
Pronounced transient red shift, linewidthbroadening and decrease of intensity
Problems:Dipole-dipole coupling in the adlayerand small concentration of „products“
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
Electron localization in adsorbate layers
Electron localization and solvation in polar adsorbate overlayers
H2O: 1.7.eV
NH3: 0.8.eV
Optical absortion spectrumSchindewolf, Angew. Chemie 80, (1968) 165
Solvated electrons in polar liquids
Courtesy: C.P. Schulz, MBI Berlin
Brief history:
Davy (1808) reports blue ammoniacontaining compounds
Weyl (1863) discovers a bluesolution of Na in liquid ammonia
Kraus (1908) attributes the bluecolor to trapped electronssurrounded by NH3 molecules
Ogg (1940) develops firstmodel for solvated electrons
Hart & Boag (1962) discoverthe hydrated electron in water
Time-resolved two-photon photoemission
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:
4.0
3.5
3.0
2.5
4.0
3.5
3.0
2.5
E-E
F[eV]
2PPE intensity
IS
SS
solvatedelectrons
2PPE
inte
nsity
4002000-200pump-probe delay [fs]
SS
solvated electrons
1 BLD O/Cu(111)2
E-E
F[e
V]
2PPE spectra as a function of time delay
analysis of solvation and localization dynamicsusing time- and angle-resolved 2PPE
2PPE experiment
e-
TOFUHV-chamber
QMS
∆t=
-platesλ 2
CCD-camera
Pinhole-doser
gas doser
Probe
∆xc
variabletime delay
tunable
Part I: Electron transfer dynamics in D2O/Cu(111)
Dynamics of photoinjected electrons in multilayers of amorphous ice
eS
eCB
hνpump= 3.92 eVhνprobe= 1.96 eV
D2O/Cu(111)4 BL
τp = 65 fs
E - EF [eV]
delay [fs]
2PP
E in
tens
ity
4.03.63.22.82.4
0
200
400
600
time-resolved spectra
Ultrafast relaxation within experimental time resolution (eCB -> eS)
Stabilisation of localized electronic state (eS) on time scale of 0.1-1 ps
two-photon-photoemission
E-E
[ eV]
4.0
3.5
3.0
2.5
E-E
F[e
V]
6004002000delay [fs]
eS
eCB
solvatedelectrons
conductionband
Energetic stabilization and population decay
3.0
2.8
2.6
e pe
ak m
axim
um: E
- E
SF[
eV]
150010005000delay [fs]
5.0 BL3.8 BL3.0 BL2.9 BL
Coverage
eS
peak shift
delay [fs]
1
10
100
2PPE
inte
nsity
8004000
τ =110 fs
eSeCB
population
eS: energetic stabilization 300 meV/ps,non-exponential decay t >100fs
eCB: population decay within pulse width
Isotope effects:very similar stabilization ratefor H2O and D2O
~50 meV shift due to differentband gap/ orientational disorder
isotope effect
delay [fs]
Dispersion and localization dynamics
Angle-resolved 2PPE spectra hν2
mek||= 2 Ekin / h2 sin(α)
TOF
e-
α
z
first moment
eS
eCB
3.0
2.9
2.8
2.7
E-
EF
[eV]
0.10.0
k|| [Å-1]
3.53.02.5
E - E F [eV]
2PP
EIn
tens
ity
α0°8°
14°18°
∆t= 0 fs
∆t= 200 fs
∆t = 0 fs
50 fs
100 fs
200 fs
300 fs
1) Electron injection intodelocalized state witinexperimenal time resolution
2) Localization within
collectivesolvation coordinate
(0)
q
E-E
F
(eV
)0
5
q1
V k qCB || 1( , )
V qS 1( )
Mechanism
Electron transfer, localization and solvation dynamics in H2O/Cu(111)
hω2
z
E-E
F
(eV
)
0
-5
5
VB
CB
Cu(111) H O2
Evac
hω1 Egap
real space
eSV qS 2( )
Electron transfer into theconduction band (CB) ofthe ice layer
Relaxation to the bottomof CB (τ
Summary
IntroductionNon-adiabatic processes at surfaces: Chemicurrents
Electron thermalization in metals
Test of the two-temperature model
Electron dynamics thin ice layers on metals
Electron injection, localization and solvation
Ru(001)
SFG
0.0001
0.001
0.01
0.1
1
2PPE
inte
nsity
1.51.00.50.0
E - EF (eV)
∆t / fs0100200300400500
Ru
2PP
E In
tens
ity
2PPE
e-+ +-
Cu
D O2
2PPE
Surface femtochemistry
Electron and phonon mediated pathways
Isotope effects and electronic friction model
Vibrational sum-frequency generation spectroscopy
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
M. Bonn, AMOLF, The Netherlands
S. Wagner, and C. Frischkorn, Freie Universität Berlin
H. Petek and S. Ogawa, Femtosecond time-resolved two-photon photoemission studiesof electron dynamics in metalsProgress in Surface Science 56, 239-311 (1998).
H. L. Dai and W. Ho, Laser Spectroscopy and Photochemistry at Metal SurfacesWorld Scientific (1995).
Literature:
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).
H. NienhausElectronic excitations by chemical reactions at metal surfacesSurface Science Reports 45, 1-78 (2002).