Theoretical cross-sections for M-changing collisions of symmetric-top molecules with rare-gas atoms

Cross-sections for collisional defocussing of hexapole beams of symmetric-top molecules have been calculated on the basis of time-dependent perturbation theory. Defocussing occurs as a consequence of changes in the M quantum number induced by the rapidly varying dipole-induced dipole potential during a long-range 'fly-by' collision. According to this model, in which the energy lost by the relaxing molecule is initially transferred to the interaction potential field and appears only later as kinetic energy of relative translation, the changes in M which occur are governed by Raman-type selection rules and intensity factors. The calculated defocussing cross-sections are in good accord with experimental results and lead to a number of predictions that have still to be tested. Calculated state-specific cross-sections for collisional relaxation of particular \J, K, M> levels do not differ very greatly from the highly-averaged cross-sections found for defocussing. These calculations, and the experiments to which they relate, provide a means of access to collision processes involving extremely small energy changes.