Small-angle ion-molecule vibrational excitation

The strong coupling correspondence principle is formulated to provide an approximation to the T-matrix elements for vibrational excitation of polyatomic molecules in rotationally sudden collisions. The theory is valid in the forward diffraction zone and at wider angles. Using the long-range form of the interaction potential this theory is applied to diatoms and linear triatoms. It is shown that there are three ranges of scattering angle in each of which different subsidiary approximations are valid. Particular application is made to proton collisions with CO, NO and N2, approximate analytic expressions being obtained for the cross section in the forward direction and at larger scattering angles. The excitation of the infrared-active modes of CO2 by protons is also considered. (author)