A lowest-order theory of vibrational excitation of polyatomic molecules upon resonant electron impact. Vibrational excitation in CH3Cl

We derive equations giving the differential cross section for excitation of vibrations by resonant electron impact in polyatomic molecules. The description 'lowest order' applies because many of the quantities are represented by Taylor's expansions in the normal coordinates with the series truncated after the first term that produces excitation. The discussion shows the importance of the phases of the scattering amplitude matrix in influencing the angular scattering in vibrational excitation. In particular, the linear Jahn-Teller effect between a doubly degenerate resonant state and a degenerate vibrational mode produces more scattering in the backward direction than in the forward direction in the case of the υ 0 to υ = 1 excitation. The theory is applied to methyl chloride, CH3Cl, giving a comparison of the results with experiment. (author)