Anisotropic intermolecular potentials from an analysis of spectra of H2- and D2-inert gas complexes

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Abstract
A method is developed for analyzing the discrete infrared absorption spectra of the van der Waals complexes formed between H2 or D2 and Xe, Kr, Ar, or Ne. Its application involves automatic nonlinear least squares fits of trial spectra calculated from realistic three-dimensional intermolecular potential models, to the experimental data. The secular determinant method used for calculating the energy levels of the anisotropic trial potentials proves to be highly reliable and relatively inexpensive. As a result, the present techniques should be readily applicable to cases where the anisotropic part of the potential is much stronger than it is here. The potential form used here is a Lennard-Jones (m, 6) function with independent long- and short-range anisotropy coefficients, and independent parameters characterizing the effect of the stretching of the hydrogen bond on the attractive and repulsive parts of the potential. The final fits concurrently used all uniquely assigned and nonoverlapping lines of both isotopes (H2 and D2) of a given complex, and all parameters (except ``m,'' which was varied manually) were allowed to vary simultaneously. The resulting potential surfaces are compared with those determined from molecular beam and relaxation time measurements.