ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
Photostimulated desorptions (PSD's) of CO, CO+, and CO− from a Pt surface are studied theoretically using Pt2–CO model cluster including image force correction. Calculations are performed by the single excitation configuration interaction and the symmetry adapted cluster (SAC)/SAC-CI methods. The PSD's of the ground state CO occur as the Menzel–Gomer–Redhead (MGR) process and those of CO+ (n cation) and excited (n→π*) CO* through the modified MGR process in which the upper repulsive potential curves are nonadiabatic; the process proceeds through a sequence of nonadiabatic transitions between the similar pertinent states embedded in the metal excited bands. The excited states as the desorption channels are characterized by the excitations from the Pt–CO bonding orbitals to the antibonding MO's: metal-adsorbate chemical bond cleavage by photons which leads to a repulsive potential is essential for the PSD. The electrostatic image force interaction plays only a minor role and the present result does not support the Antoniewicz model. The calculated excitation-energy thresholds for the CO, CO+, and CO* desorptions are 1.6∼2.6, 11.3, and 11.3–12.7 eV, respectively, which explains the energy thresholds and the fluence dependencies of the incident laser in the PSD experiments. On the other hand, the PSD giving CO− would occur with the energy range of 6.2–8.2 eV, one to two photon energy of the 193 nm (6.4 eV) laser. Since the upper nonadiabatic potential curves have shallow minima, in this case, the lifetime of the CO− species would be larger than those of the CO+ and CO* species. The present study clarifies the electronic structures of the desorbed CO+, CO−, and CO* species, which have not been identified experimentally. © 1996 American Institute of Physics.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.470796
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