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SFB 450 Colloquium 1/21/2003 Towards ultrafast
control of adsorbate reactions 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
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Photoreactions at metal surfaces
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Typical energy flow in a surface photoreaction

Ru
Tel gtgt Tph
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Non-coherent control CO O on Ru(001)

Femtosecond photochemistry CO oxidation vs.
desorption
? oxidation electron mediated ? strong
dependence on pulse-pulse delay ? fast

• ? desorption phonon mediated
• ? weak dependence on pulse-pulse delay
• ? slow
• conventional thermal desorption
• Bonn et al., Science 285, 1042 (1999)

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Reaction mechanism of CO oxidation on Ru(001)
? makes use of different time scales of
electronic and lattice temperature transients
? non-coherent scattering processes destroy
temporal coherence betweensubsequent excitation
steps
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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 COO on Ru(0001) ?
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Model catalyst silver nanoparticles
Optical field enhancement by plasmon excitation
(1,1)-resonance
(1,0)-resonance
? 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
(1,1) (1,0)
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Goal
Controlled photochemistry of adsorbed molecules
on silver nanoparticles
direct excitation ? wave packet dynamics
? combine direct and indirect excitation
? influence temperature transients by choice of
substrate, particle size
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Preparation of silver nanoparticles
? evaporation of Ag atoms onto quartz substrat,
Volmer-Weber growth
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Experimental setup
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Proposed reactions
? metal carbonyl dissociationhappens on most
substrates via direct 1-photon excitation around
300 nm(W. Ho, in Desorption induced by
electronic transitions, DIET IV, Springer,
Berlin, 1990)
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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.
COO/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
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Responsible
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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)