Adsorption-state-dependent subpicosecond photoinduced desorption dynamics

Femtosecond laser excitation has been used to initiate desorption of molecular oxygen from the (111) surface of Pd and to study the adsorption-state dependence of the substrate-adsorbate coupling. The relative populations of the two chemical states, peroxo (O2(2-)) and superoxo (O2-), were varied by...

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Bibliographic Details
Published in:The Journal of chemical physics 2007-06, Vol.126 (21), p.214709-214709
Main Authors: Szymanski, Paul, Harris, Alex L, Camillone, 3rd, Nicholas
Format: Article
Language:English
Subjects:
ACTIVATION ENERGY
ADSORPTION
CHEMICAL STATE
DESORPTION
ELECTRON-PHONON COUPLING
ELECTRONS
EXCITATION
INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY
LASER RADIATION
MATERIALS SCIENCE
OXYGEN
PALLADIUM
PEROXIDES
PHONONS
PHOTOCHEMISTRY
PHOTONS
PULSED IRRADIATION
SURFACES
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Summary:Femtosecond laser excitation has been used to initiate desorption of molecular oxygen from the (111) surface of Pd and to study the adsorption-state dependence of the substrate-adsorbate coupling. The relative populations of the two chemical states, peroxo (O2(2-)) and superoxo (O2-), were varied by changing the total coverage. Two-pulse correlation measurements exhibit a dominant 400 fs response and a slower 10 ps decay that are relatively independent of the initial O2 coverage. In contrast, the photodesorption yield and the nonlinearity of the fluence dependence show a systematic coverage dependence. The coverage-independent subpicosecond response indicates that the photoinduced desorption from the two states is driven primarily by the same electron-mediated mechanism, while the coverage dependence of the yield indicates that the desorption efficiency from the superoxo state is greater than that from the peroxo state. These results are discussed in the context of the electron-phonon two-temperature model with an empirical adsorbate-electron frictional coupling that depends on both the electronic temperature and the activation energy for desorption. With a coupling strength that decreases as the activation energy decreases, the trends with varying coverage, absorbed fluence, and time delay can all be reproduced. The model is consistent with a transition from a resonantly enhanced (diabatic) regime to an adiabatic regime as the system relaxes, accounting for the biexponential correlation behavior.
ISSN:0021-9606
1089-7690
DOI:10.1063/1.2735594