Axially nonadiabatic channel treatment of low-energy capture in ion-rotating diatom collisions

The quantum version of an axially nonadiabatic channel (ANC) approximation, introduced in an earlier article for the calculation of complex-formation cross sections and rate constants in ion-diatom collisions [Maergoiz, A. I.; Nikitin, E. E.; Troe, J.; Ushakov, V. G. J. Chem. Phys. 2002, 117, 4201-4213] is tested against accurate quantum results. Cross sections and rate constants are determined for several representative systems with the participation of a diatom in the state j = 1, assuming various long-range potentials between the collision partners, such as anisotropic ion-induced dipole, second-order charge-permanent dipole, and first-order charge-quadrupole interaction. The ANC approximation well reproduces accurate quantum results in the perturbed rotor limit, while the standard quantum adiabatic channel (AC) approximation fails at low energy due to neglect of Coriolis coupling. However, the low-energy extrapolation of classical adiabatic channel results (ACCI) provides a reasonable approximation both to accurate and quantal ANC results down to collision energies when only few partial cross sections determine the total capture cross section. This unexpected feature of the ACCI approximation is due to two effects: (a) an artificial simulation of tunneling transmission and overbarrier reflection at centrifugal barriers by introducing a continuous distribution over total angular momenta and (b) a slight effective lowering of the centrifugal barriers compared to centrifugal barriers within the AC model. Low-temperature quantum rate constants are also presented.