Theoretical studies of molecules adsorbed on solid surfaces: I. Electronic spectra

The band shape of electronic spectra for a diatomic molecule adsorbed on a surface is studied using the adiabatic approximation. In this approximation, the Schrödinger equation for the total system is separated into those for electronic motion, intramolecular vibration, and low frequency motions (e.g., hindered molecular rotation and lattice vibration). Our theory is applied to the system of an admolecule on the (001) surface of fcc monatomic crystals. We determine the line shape of a vibronic transition for both chemisorption and physisorption cases. In chemisorption cases, a side band due to the low frequency motions can appear in the line shape of a vibronic transition. On the other hand, in physisorption cases, the line shape is a single peak. Moreover, we suggest theoretically a new experimental technique for exciting adsorbed molecules; for this purpose, we derive equations giving the cross sections for the electronic excitation of admolecules by using ion impact at grazing incidence. The admolecules can be excited by this method; the subsequent light emission can give information on the orientation of the admolecule on the surface and on the admolecule-surface species interaction.