M-DEPENDENCE OF COLLISIONAL TRANSFER OF ROTATIONAL ENERGY IN METHANOL

Abstract

A steady-state microwave double resonance experiment on the $(4_{O} \leftarrow 3_{1})_{p} - (2_{1} \leftarrow 3_{0})_{s}$ four-level system in methanol is described in which collisional transfer is observed between individual M-substates of pump and signal levels split by a Stark field. For pure $CH_{3}OH$, the observed collisional preferences follow dipolar selection rules, $\Delta M = 0, \pm 1$, and the relative intensities of the signals are consistent with a simplified dipole-dipole model of the collisional interaction. For dilute mixtures of $CH_{3}OH$ in excess He and $H_{2}$, collision-induced transitions with $|\Delta M| > 1$ are seen, which give evidence for high-order terms in the collisional interaction with these foreign gas partners.