Transition rates between selected vibrational states in methyl halides; influence of anharmonicities

A semiclassical model of collision induced vibrational relaxation is discussed in terms of an effective collision mass for different values of vibrational energy release. Selected one, two and three quantum transitions of the methyl halides upon collision with rare gases are evaluated in the presence of resonant and nonresonant anharmonic couplings. It is found, that due to the anharmonic coupling the rates between CH stretching modes and the overtones of the CH bending modes become as large as the transfer rates between two CH stretching modes. This is in qualitative agreement with experiments. Without the anharmonic coupling they differ by two orders of magnitude.