Martin WolfDepartment of Physics, Freie Universität Berlin

IMPRS Block Course “Dynamic Processes on Surfaces”

Ultrafast laser spectroscopy and surface dynamics

Question: How can we catch the motion of objects?

…a fast camera is enough!

Stroboscopic investigation of motion and structural changes

Surface femtochemistry

Surface dynamics with electronic friction

Outline

IntroductionNon-adiabatic processes at surfaces: Chemicurrents

Example: Associative desorption of H2 from Ru(0001)

Bi

Attosecond laser spectroscopy

Electron streak camera and Auger decay

High harmonic and attoseond pulse generation

Time-resolved probe of structural dynamics

Time-resolved x-ray diffraction

Femtosecond laser spectroscopy

Non-thermal melting and coherent phonon excitation

Time-resolved photoelectron spectroscopy

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

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

Coupling between nuclear 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 in gas-surface interaction

metal semiconductor

e-h pair

e-

Gergen et.al., Science 294, 2521 (2001)

Chemicurrent observed forvarious adsorbates

Energy dissipation via e-h pair excitation

electronic excitationsplay an important roleduring adsorption

mechanism of e-h pair excitation ?

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- Exoelectron emission

Chemiluminescence

EF

Electronic friction

Electronic friction of adsorbates

Example: Increase of electric resistivityin a thin metal film upon Xe adsorption

-V Xe

~1 ML Xe/Ag film

Newns-Anderson picture providesa mechanism for electronic friction

vibrational damping at metals

surface femtochemistry

Brandbyge, et al, PRB 52, 6042 (1995); B. Persson, in Sliding Friction (Springer, 1998)

Femtochemistry at metal surfaces

substrate-mediated excitation mechanism dominates

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

non-thermal reaction mechanism:separation of time scales of energy flow

Reaction mechanism

electron-mediated energy flow

Desorption Induced byMultiple Electronic Transitions

DIMET -

fs-laser excitation

Lisowski et al., Appl. Phys. A 78, 165 (2004)

electron thermalization

τthermalization << 100 fs * MGR model (Menzel, Gomer, Redhead)

*

electrons in a metal

Experimental InvestigationsDesorption:• NO and O2/Pd• CO/Cu, CO/Pt• O2/Pt • CO/NiOReactions:• CO + O2/Pt• CO + O, C + O, H+H/Ru

Misewich, Loy, HeinzTom, PrybylaHo MazurStephenson, Richter, CavanaghBonn, Wolf, ErtlZacharias, Al-Shamery, FreundDomen

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

associative H2 desorption from H/Ru(0001)

ultrashort

laser pulse

Femtosecond laser induced desorption of H2 by 100 fs, 10 mJ/cm2 pulses

Remarkably high translational energy (<Etrans>/2k ~ 2000 K)

1x1 H/Ru(001)

Denzler et al. Phys. Rev. Lett. 91, 226102 (2003) & J. Phys.Chem. B 108, 14503 (2004)

2 pulse correlation measurements

Ultrafast response indicates coupling to hot electron transient

FWHM = 1.1 ps

first

shot

yiel

d[a

.u.]

pulse-pulse delay [ps]

H2exp. data

0

-10 -5 0 5 10

phonon-mediated: slow responseelectron-mediated: fast response

0

160140120100806040200

0

exp. data

model

power law fit

yield weighted fluence [J/m2]fir

stsh

otyi

eld

[a.u

.]

isotoperatio

H2

D2

Y ~ <F>3fir

stsh

otyi

eld

[a.u

.]

pulse-pulse delay [ps]

H2exp. data

0

-10 -5 0 5 10

model

FWHM = 1.1 ps

Electronic 1D friction model provides consistent description:• 2-pulse correlation• fluence dependence• isotope effects

Ea = 1.35 eV

ηel = τel = 180 fs for H2-1

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

Brandbyge et al., PRB 52, 6042 (1995)

Coupled heat baths with Tel, Tph, Tads

friction coefficient activation energy

ηel

Tads∫Reaction rate ~ Ea exp(– Ea / kTads)

Electronic friction model

ηel = τel-1

Energy partitioning

Energy partitioningwithin the D2 product

Resonance EnhancedMultiphoton Ionization (REMPI)

helicoper molecule cartwheel molecule

|i>

|r>

IP

VUV

UVscan VUVrovibrational population distribution

change VUV polarizationmolecular alignment

Wagner et al., Phys. Rev. B 72, 205404 (2005)

cooperation with H. Zacharias (Münster)

Experimental setup

VUV conversion efficiency 10-6 330 nm: 8.5 mJ/pulse

532 nm: 330 mJ/pulse266 nm: 4.5 mJ/pulse

Reaction trigger: fs-laser + state-selective detection: tunable VUV sourceReaction trigger: fs-laser

State-resolved detection of desorbing D2

No Boltzmann-like rotational population distribution

⟨Erot⟩ = 80 meV⟨Evib⟩ = 100 meV

⟨Etrans⟩ = 430 meV

N ~ exp-E kBT

Wagner et al., Phys. Rev. B 72, 205404 (2005)

Reaction dynamics Detailed picture of reaction mechanism and energy flow

molecular alignment

A0 = 0.3preferentiallyhelicoper-like

(2)

quadrupole alignment parameter -1 ≤ A0 ≤ 2(2)

coupling times and activation energy

hot electron driven high translational energy

unequal energy partitioning

predominantly translational excitation

⟨Etrans⟩ : ⟨Evib⟩ : ⟨Erot⟩ = 5.5 : 1.5 : 1

energy balance

good agreement with 1D friction model

⟨ED2_flux⟩ = 160 meV = kB Tadsfriction

PES topology (barrier location)

multidimensional friction / energy transfer

1D model fits energy balance: Why ?

Theory: Molecular dynamics with electronic friction

Head-Gordon & Tully, JCP 103, 10137 (1995)

µd = – ηdd d – ηdz z + Fd(t)∂V∂d

.. . .

µz = – ηzz z – ηdz d + Fz(t)∂V∂z

.. . .

µq = – ηq q + Fd(t)∂V∂q

.. .

Equations of motion:

cooperation with A. Luntz (Odense) and M. Persson (Liverpool)

Fluctuating forces F(t) by frictionalcoupling to electron temperature

( ) ( ) ( )2i i B e iiF t F t k T t tη δ′ ′= −

(fluctation dissipation theorem)

Friction coefficients and PES

2x2 unit cell for DFT (LDA)

≈ 60 meV

PES:Friction coefficients (TDDFT):

Phys. Scr. 29, 360 (1984)JCP 119, 4539 (2003)

reaction path S

2-pulse correlation & fluence dependence

Luntz, Persson, Wagner, Frischkorn, Wolf, JCP (submitted)

“First principle” model: V2D(z,d) AND η(z,d) no adjustable parameter

EXCELLENT agreement with experimenttime scale of energy transferfluence nonlinearity of yieldisotope effect

success of 1D model ?

multi-dimensional friction

small S: ~ isotropic coupling

near V*: ηdd ≈ 3 ηzz

origin of unequal energy partitioning ???

crucial time span0 < t < 1 ps

excitation occurs only whileelectrons are still hot

time evolution

reaction path S

Trajectories: dynamics on ground PES

F = 140 J/m2 F = 140 J/m2

rapid interchange between z and d(”thermalization”)

sufficient energy is a prerequisite,

... but not everything

dynamics on ground state PES

reaction

reaction no reaction

Summary I

IntroductionNon-adiabatic surface dynamics: Chemicurrents

Bi

Time-resolved photoelectron spectroscopy

Surface femtochemistry

Femtochemistry driven by hot electron excitation

Example: Associative desorption of H2 from Ru(0001)

Multidimensional dynamics with electronic frictions

metal semiconductor

e-h pair

e-

Time-resolved probe of structural dynamics

Time-resolved x-ray diffraction

Femtosecond laser spectroscopy

Non-thermal melting and coherent phonon excitation