Computational tests of angular momentum decoupling approximations for pressure broadening cross sections

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Abstract
The utility of several approximate scattering methods for predicting collision induced spectral pressure broadening has been tested by comparison with accurate close coupling results. In particular, broadening of the pure rotational spectra of HD, HCl, CO, and HCN—all perturbed by low energy collisions with He atoms—has been computed using the effective potential formalism of Rabitz, the decoupled l-dominant approximation of DePristo and Alexander, and the jz-conserving coupled states method of McGuire and Kouri. For this last method, pressure broadening cross sections have been obtained with the new, correct expression recently derived by Goldflam and Kouri as well as with an earlier formalism based on an incorrect labeling of the scattering matrices. These methods were found to be generally disappointing for predicting pressure broadening with the exception of the new, correctly formulated jz-conserving coupled states method which was found to agree quantitatively (better than 5%) with close coupling values for all cases studied.