Vibrational relaxation of carbon dioxide at LiF(100)

Abstract

The vibrational relaxation of CO2 at LiF(100) has been investigated by monitoring infrared fluorescence from vibrationally excited molecules under conditions where they are relaxed primarily by collisions with the solid surface. The relaxation probabilities are found to be 0.65±0.10 at room temperature and 0.35±0.10 at 450 K. In order to understand better the vibrational relaxation results, angular distributions of CO2 scattered from LiF(100) were measured with a molecular beam scattering apparatus. At slow incident beam velocities, the trapping probability of CO2 at LiF(100) is essentially unity. Thus, in the vibrational relaxation measurements, where the incident velocity is even slower than in the scattering experiments, vibrational relaxation is preceded by trapping. Possible mechanisms for relaxation are discussed. Excitation of phonons in the solid and transfer of energy to other degrees of freedom of the molecule (i.e., translation, rotation, and other vibrational modes) are both plausible relaxation channels.