Energy transfer in NH3–He collisions

The electron gas intermolecular potential for NH3–He developed previously is shown to be inadequate for a quantitative description of energy transfer in this system by comparing with available experimental data—pressure broadening, double resonance, and beam scattering. A new potential is constructed by combining the Hartree–Fock results of Davis, Boggs, and Mehrotra with semiempirical long-range induction and dispersion terms. The resulting potential is shown to be in harmony with beam scattering and pressure broadening measurements. It is found that the quadrupole-induced–dipole interaction, which has been invoked previously to account for pressure broadening in this system, is not, in fact, as effective as the short-range anisotropy. Although results are in general accord with double resonance data, a few unresolved discrepancies remain. It is suggested that these are due to improper consideration of the dependence of cross sections on degenerate magnetic quantum levels in the experimental analysis. Using this new intermolecular potential state-to-state rate constants are presented for temperatures to 300 K.