Knowledge Bank

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 $H2CO1$. 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 (Fig. 1), and in 4 level DR to adjacent levels. Our analysis of M-changing collisions uses the statistical tensor reduction of the equations for the time-evolution of the density matrix, with the observed rates expanded in an extended ECS / tensor opacity form to identify the simplest model consistent with the observed polarization dependence of our 3 and 4 level double resonance signals. [FIGURE]