Relative contributions of ΔJ = 2 and ΔJ = 1 transitions in rotationally inelastic collisions of polar molecules

Previously developed first- and second-order semiclassical perturbation theory is employed to treat rotationally inelastic collisions between pairs of linear molecules. The dependence of the cross sections σ (ΔJ = 1) and σ(ΔJ = 2) on the rotational constants, dipole and quadrupole moments, and reduced masses of the colliding molecules is investigated. It is shown that the rotational constant has a dominant role in determining the relative magnitude of the cross sections ΔJ = 2 compared to ΔJ = 1. Much of the observed cross section behavior can be explained through collisional resonance effects. The conditions for significant ΔJ = 2 transitions are discussed and illustrated with rotational transitions in the molecules HF, HCl, DCl, HBr, OCS, HCN, ClCN, and BrCN. Each of these molecules interacted with a thermal bath of molecules of the same species. Results are also presented for the systems HCl:Cl and DCl:Cl where the first molecule in each pair is the observed species. The resulting cross section ratios σ(ΔJ = 2)/σ(ΔJ = 1) in these studies ranged from 0.14 for HF to 0.73 for DCl:HCl.