Theoretical study on the photostimulated desorption of CO from a Pt surface

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.