Rate constants for rotational excitation in NH3–He collisions

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Abstract
The semiclassical method (classical translational motion and quantum mechanical rotational motion) is used to calculate cross sections and rate constants for (J?3, K) → (J?4, K) rotational excitations in NH3–He collisions on Green’s uniform electron gas (Gordon–Kim) potential surface. The assumptions involved in the use of the rigid rotor approximation for the inversion motion are examined in some detail. Agreement between semiclassical and accurate close‐coupling cross sections is good, comparable to the agreement between the coupled‐states and the close‐coupling cross sections. A ’’semiclassical coupled‐states’’ approximation was found to give reliable cross sections except at low energies. Microwave double resonance ΔI/I values were calculated from the rate constants and compared with experiment. Good agreement was found for the Δk=3 transitions, but poor agreement for the Δk=0 transitions. Possible explanations for the discrepancies are discussed.