Rotational excitation of symmetric top molecules by collisions with atoms. II. Infinite order sudden approximation

Publisher Website
Google Scholar
Abstract
The infinite order sudden approximation is extended to rotational excitation of symmetric tops by collisions with atoms. The formalism is developed first for ’’primitive’’ or ’’one‐ended’’ tops. The proper parity adapted linear combinations which describe real rotors are then considered. Modifications needed for asymmetric rigid rotors are noted briefly. For all of these cases generalized spectroscopic relaxation cross sections are discussed; these include degeneracy averaged state‐to‐state integral cross sections and pressure broadening cross sections as special cases. In the IOS formalism these cross sections factor into dynamical terms which contain all the system dependent collision information and spectroscopic coefficients which contain the dependence on rotational levels and angular momentum coupling. This factorization is similar to that obtained for linear rotors, but differs insofar as the dynamical factors cannot all be expressed in terms of single state‐to‐state cross sections and insofar as the generalized cross sections are no longer necessarily real quantities. IOS calculations for NH3–He and H2CO–He have been performed to compare with earlier, more accurate calculations. Both of these systems provide fairly severe tests since they have rather widely spaced energy levels with only a few levels accessible at thermal energies whereas the IOS approximation should be best for the opposite case of many closely spaced levels. Nevertheless, the IOS approximation is found to provide a reasonably accurate description for these systems. Finally, implications of the IOS factorization for the interpretation of microwave double resonance experiments for these systems is considered.