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).