TORSIONALLY INELASTIC-COLLISIONS BETWEEN A NEAR-SYMMETRICAL TOP MOLECULE AND A STRUCTURELESS ATOM

The close-coupling formulation is presented for collisions of a structureless atom with a near-symmetric top exhibiting internal rotation, such as methanol. The molecule is approximated as a symmetric top whose internal rotation axis coincides with the symmetry axis. The K doubling arising from the asymmetry is taken into account only to first order. Both rotational and torsional inelasticity are considered, but the molecule is considered to be rigid with respect to all other vibrational degrees of freedom. Expressions are given for the matrix elements of the interaction potential between any two rotational-torsional states. It is shown that A <--> E and E1 <--> E2 collisional excitation is forbidden. The infinite order sudden (IOS) approximation is extended to systems exhibiting internal rotation by applying the sudden limit to the torsional motion as well as to the overall rotation. Based on the expressions for the IOS cross sections, it is shown that a propensity rule holds for transitions elastic in J or in K between two A +/- doublets similar to that governing transitions between K doublets in asymmetric top molecules. It is also shown that in the low-barrier limit, torsionally inelastic cross sections depend only on DELTA-n, the change in the angular momentum of the internal rotor, and not on the particular initial and final n. The IOS factorization expression shows that in the low-barrier limit, all of the dynamical information for E <--> E transitions is contained in the cross sections for excitation out of the J = 0, K = 0, v = 0 ground state.