Adiabatic theory of infrared laser-induced predesorption of CO from a NaCl (100) surface

Laser-induced vibrational/rotational predesorption of CO from a NaCl(100) surface is theoretically studied using the master equation in an adiabatic representation. Extending our previous treatment (Y. Ohtsuki, T. Kato, Y. Fujimara and S. H. Lin, J. Chem. Phys., 1997, 106, 4339), we adopt a three- dimensional model (CO stretching, CO-surface stretching and bending) with an adsorption potential proposed by Ben Ephraim et al. (A. Ben Ephraim, M. Folman, J. Heidberg and N. Moiseyev, J. Chem. Phys., 1988, 89, 3840), while surface motion is taken into account by using a relaxation operator. Since the adiabatic approximation works well to separate the CO stretching and CO-surface vibrations, the desorption, which is expressed as a non-adiabatic transition from bound states to the desorption continuum, is quantitatively evaluated within the perturbative treatment. The experimentally observed rate is quantitatively reproduced by the present calculations. It is shown that both vibrational and rotational predesorption make important contributions to the desorption of CO/NaCl. The calculated results show that the predesorption rate is strongly dependent on the surface temperature and that the rate changes by a factor of 12 in the range of 50 K ≤ T ≤ 150 K. Inter-site interaction effects on predesorption are also examined by using a two-site model with phenomenological energy transfer rates. The energy migration among the adsorption sites can effectively increase the lifetime of the excited CO, resulting in an increase in predesorption probability.