sfb 450 colloquium – 1/21/2003 towards ultrafast control of adsorbate
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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)
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