SFB 450 Colloquium – 1/21/2003 Towards ultrafast control of adsorbate

1. 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

2. Photoreactions at metal surfaces Potential advantages:  orientational ordering  co-adsorbate systems  catalytic properties  substrate-mediated reactions Main problem:  decay of electronic excitation via coupling to substrate electron bath

3. Typical energy flow in a surface photoreaction Substrate-mediated mechanisms dominate multiple scattering processes completely destroy coherence Mechanisms and time scales of energy transfer after optical excitation Model of coupled heat baths for electrons and phonons transient non-equilibrium: Ru Tel>> Tph

4. Non-coherent control: CO + O on Ru(001) temporal evolution after fs laser pulse: 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)

5. Reaction mechanism of CO oxidation on Ru(001) Phenomenological: Friction model Microscopic: Reaction by multiple short-lived electronic excitation coupling time: tel=(0.5±0.1) ps Non-coherent control of CO oxidation/desorption on Ru(001):  exploitstemperature difference between metal electrons and lattice upon ultrafastexcitation  makes use of different time scales of electronic and lattice temperature transients  non-coherent: scattering processes destroy temporal coherence betweensubsequent excitation steps

6. 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)

7. 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)

8. Goal:  use different time scales of direct, electron-, and phonon-mediated excitation 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

9. Preparation of silver nanoparticles  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: F. Stietz und F. Träger, Philos. Mag. B 79 (1999) 1281  evaporation of Ag atoms onto quartz substrat, Volmer-Weber growth

10. Experimental setup Feedback loop

11. Proposed reactions  steady state reaction:desorption/isomerization of 1-epoxy-3,4-butene (EpB)  happens thermally under favorable conditions  future goal: bimolecular reaction (synthesis)  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)

12. 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

13. Responsible Martin Wolf Arthur Hotzel David Starr Sebastian Kwiet Alexander Grujic

14. 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)