Velocity-averaging effects in beam-gas dynamics experiments

The potential magnitudes of experimentally observable effects of spatial alignment on the reactivity of target molecules in "beam-gas" experiments have been investigated by performing model calculations. The target gas is assumed to consist of diatomic molecules prepared from a sample in thermal motion by pumping with a linearly polarized laser on a selected IR transition. A collimated effusive atomic beam impinges on the target gas. A general analytical form of the distribution in magnitudes and laboratory frame directions of the collision velocity in such a beam-gas experiment has been derived, incorporating the effects of the thermal motion of the target. This distribution is combined with the known properties of the IR pumping process to deduce the distribution of collision velocity magnitudes and directions relative to the diatomic molecular axis. The significance of these results is investigated by assuming simplified model forms for the intermolecular potentials controlling reactivity as a function of the direction of approach. The stereochemical effects for different potentials have the anticipated signs and magnitudes, including the maximum effect for surfaces with a preferred collinear geometry and a vanishing effect for those bent near the "magic angle". It is concluded that velocity averaging in itself was not responsible for the absence of a measurable effect in the typical case of Sr + HF, for which prior experimental data exist.