SFB 450 Colloquium – 1/21/2003

Towards ultrafast control of adsorbatereactions on silver nanoparticles

Arthur Hotzel, FU Berlin, Teilprojekt A6

Incoherent control of photoreactions on metal surfaces

How to make the step to coherent control

Our model catalyst: silver nanoparticles

Proposed model reactions

silver nanoparticle

dielectric substrate

Potential advantages:

orientational ordering

co-adsorbate systems

catalytic properties

substrate-mediated reactions

Photoreactions at metal surfaces

reaction coordinate

ener

gy

Main problem:

decay of electronic excitation via

coupling to substrate electron bath

Typical energy flow in a surface photoreaction

Substrate-mediated mechanisms dominate

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

multiple scattering processes completely destroy coherence

Mechanisms and time scales of energy transfer after optical excitation

Model of coupled heat baths for electrons and phononstransient non-equilibrium: Tel >> Tph

Rufemtosecond

excitation

electrons Tel p h o n o n s T p h

adsorbate nuclear degree of freedom Tads

met

al ~1 ps0.1-1 ps

>1 ps

~0.1 psreaction

(d irectabsorption)

heat transport

Non-coherent control: CO + O on Ru(001)

C

OO

t

Tel Tph

CO

CO 2

QMS

-200 -100 0 100 200

x24 CO2

CO

first

shot

yiel

d(a

.u.)

pulse-pulse delay (ps)

200 400 600

CO2

<Fluence> (Jm -2)200 400 600

CO

<Fluence> (Jm-2)

3 psFW H M

20 psFW H M

3 psFW H M

Femtosecond photochemistry:CO oxidation vs. desorption

oxidation: electron mediated strong dependence on pulse-pulse delay fast

2000

4000

6000

Tel

Tph

tem

pera

ture

0 5 10 15

0.0

0.2

0.4

0.6

0.8

1.0 COCO2

norm

aliz

edra

te

time (ps)

temporal evolution after fs laser pulse:

desorption: phonon mediated weak dependence on pulse-pulse delay slow~ conventional thermal desorption

Bonn et al., Science 285, 1042 (1999)

Reaction mechanism of CO oxidation on Ru(001)

Phenomenological:Friction model

electrons Tel

reaction coordinate Tads

friction parameter

coupling time:el=(0.5±0.1) ps

Microscopic:Reaction by multiple short-lived electronic excitation

reaction coordinateen

ergy

ne utra l g ro undsta te

O - sta te

Non-coherent control of CO oxidation/desorption on Ru(001):

exploits temperature difference between metal electrons and lattice upon ultrafast

excitation

makes use of different time scales of electronic and lattice temperature transients

non-coherent: scattering processes destroy temporal coherence between

subsequent excitation steps

Towards coherent control on metal surfaces

For a reaction by well-defined intra-molecular excitations of adsorbed molecules:

Increase efficiency:

increase lifetimes of electronic excitations

decouple intramolecular excitations from metal substrate (decrease orbital overlap)

larger molecules/spacers

use substrate with smaller electronic density of states

noble metals

Enhance direct pathways vs. indirect (substrate-mediated) pathways:

increase electric light field at surface vs. heat dump into substrate electron system

use photon energies below onset of interband (d-band) transitions

noble metals

use additional field enhancement

Don't do CO+O on Ru(0001)

Model catalyst: silver nanoparticles

Optical field enhancement by plasmon excitation:

(1,1)-resonance (1,0)-resonance

h

extin

ctio

n (1,1) (1,0) Plasmon resonances at ~2 - 3.5 eV

(for silver)

field enhancement at surface of

nanoparticles, factor ~5 - 30

Kreibig/Vollmer, Optical Properties of Metal

Clusters, Springer, Berlin, 1995

use different time scales of direct,

electron-, and phonon-mediated excitation

10 ps10 fs 1 ps

Goal:

Controlled photochemistry of adsorbed molecules on silver nanoparticles

p lasm ons electronsTel

phononsTph

adsorbate states

<10fs 1ps

incoherentcoherent

adsorbates

direct excitation

wave packet dynamics

combine direct and indirect excitation

influence temperature transients by

choice of substrate, particle size

Preparation of silver nanoparticles

evaporation of Ag atoms onto quartz substrat, Volmer-Weber growth

too oblate o.k. too spherical

completeevaporation

BEFORE

AFTER

F. Stietz und F. Träger,

Philos. Mag. B 79 (1999) 1281

laser shaping: irradiation with 532 and 355 nm

selective excitation of clusters with corresponding shape and size

atom evaporation, "shaping"Extinction spectra of Ag nanoparticles on quartz:0.6

0.5

0.4

0.3

0.2

0.1

0.0

Ext

inct

ion

4.54.03.53.02.52.01.5

Photon Energy [eV]

p-pol. light: before andafter laser shaping

Experimental setup

N d:YAG10 H z

SH G +TH G

w hite light

Ag evap.

QMSextinctionspectrosc.

load lock

quartzsam ple

gasdoser

U H Vcham ber

clu

ste

r sh

ap

ing

Co

her

ent

Co

ron

a

Fem tolasers Fem topower Pro

C oherentVerd i 5W

Ti:sapph.oscilla tor

Pockelsce ll

stre tcher (g lass)

Ti:sapphire

com pression

BBOSH G

program m ableR Fgenerator

FastliteDazzler

AO crysta lpulse shaping

BBO for D FG400+800 nm

beamdiagnosis

Feedbackloop

Proposed reactions

metal carbonyl dissociation:

happens on most substrates via direct 1-photon excitation around 300 nm(W. Ho, in Desorption induced by electronic transitions, DIET IV, Springer, Berlin, 1990)

EpB

Ag

O

C steady state reaction:

desorption/isomerization of 1-epoxy-3,4-butene (EpB)

happens thermally under favorable conditions

future goal: bimolecular reaction (synthesis)

Summary

Photoreactions at metal surfaces: fast loss of coherence due to substrate-mediated

scattering processes

Non-coherent control of reaction branching ratios: use different temperature

transients of substrate electron and phonon systems, e.g. CO+O/Ru(001)

Strategy for coherent control:

decrease adsorbate-substrate coupling

enhance direct excitation cross section vs. substrate-mediated channels

Silver nanoparticles:

plasmon-mediated field enhancement

preparation and laser shaping

Proposed model reactions:

metal-organic adsorbates

steady state reaction (EpB)

bi-molecular reaction

Responsible

Martin Wolf Arthur Hotzel

David Starr

Alexander GrujicSebastian Kwiet

Acousto-optic programming dispersive filter (Fastlite Dazzler)

birefringent crystal (TeO2) + transducer

RF wave travels collinearly with light beam

ultrafast light pulse sees stationary spatial modulation of lattice distortion

light is scattered out of ordinary beam into extraordinary beam by RF pulse.

output pulse is essentially the temporal convolution of the input pulse with the RF

pulse shape.

Verluise et al., Optics Letters 25, 575 (2000)