Rate constants for rotational excitation in NH3--He collisions

The semiclassical method (classical translational motion and quantum mechanical rotational motion) is used to calculate cross sections and rate constants for (J< or =3, K) → (J< or =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