On the amount of information in rotational relaxation experiments with application to microwave transient T1 and T2 rates

Abstract

A variety of gas phase rotational relaxation phenomena are related at the microscopic level by intermolecular forces and binary collision dynamics. By making the infinite order sudden (IOS) approximation to collision dynamics, the different phenomena can be expressed in terms of a few fundamental ’’dynamical’’ parameters and these can be chosen conveniently as the state-to-state rates out of the lowest level. Microwave transient T1 and T2 rates for a linear rigid rotor perturbed by an atom are considered explicitly. For systems where the IOS approximation is valid, i.e., where many rotational levels are energetically accessible, it is found that T1 and T2 measurements are expected to give rather similar rates, regardless of the spectral transition and in the presence or absence of an external (Stark) field; further, these measurements basically reflect the total rate of collisional excitation out of the lowest level. In principle, the small changes in T2 between different spectral lines or between Stark split sublevels provide information about specific, even Δj state-to-state rates. Similar T1 data provide, in addition, information about odd Δj transitions. As an example, IOS calculations are presented for OCS perturbed by Ar and He; these give relaxation cross sections in excellent agreement with the earlier semiclassical calculations of Liu and Marcus. Finally, methods for inverting experimental data to obtain state-to-state rates are considered.