Collision Excitation of Molecular Vibrations in Halogen-Substituted Methanes

Abstract

Departures from the equilibrium between vibrational and translational energies in gases are characterized by relaxation times which depend upon the ease with which vibrations may be excited or de-excited by collisions. The de-excitation process has been discussed by various authors on the basis of the relative velocity of approach of two colliding molecules and on the basis of the relative energy of approach. The dispersion of ultrasound has been measured in fourteen halogen-substituted methanes and compared with existing data. Binary collisions were found to be responsible for the excitation of vibrations, and all the gases were found to have a single relaxation time indicating a strong intermodal coupling. The probability, in a collision, of exciting or de-exciting molecular vibrations was found to depend upon the relative energy of approach of the molecules. Molecules with the same number of halogen substitutions resemble each other in this respect. The data suggest that the colliding molecules might form a complex molecule with a short lifetime during which energy exchange takes place readily.