Reorientation due to self-collisions in ground-state methane

Collision-induced changes in the space-fixed body-frame angular momentum projection, M, occur in all systems, within limits imposed by the isotropy of the collision environment. Their significant effect on the relationship between calculated inelastic rates and observed linewidths and state-to-state rates is often neglected. Because of the properties of the multipole, or tensor opacity, expansion of the relaxation matrix, M-memory on collision is expected to be greatest for low-order multipole interactions, as in the case of H2CO. The lowest order permanent multipole in CH4 is the octopole. Surprisingly, CH4 ground state collisions in the E manifold (where several levels are missing due to nuclear spin statistics) show strong M-memory in low J levels, both in 3 level DR, and in 4 level DR to adjacent levels. The authors analysis of M-changing collisions uses the statistical tensor reduction of the equations for the time-evolution rates expanded in an extended ECS/tensor opacity form to identify the simplest model consistent with the observed polarization dependence of the 3 and 4 level double resonance signals