Rotational excitation of NH3 and ND3 due to He atom collisions

Abstract

Quantum close-coupling and coupled-states approximation scattering calculations for rotational energy transfer of rotationally excited NH3 and ND3 due to collisions with He are presented. Calculations were performed for collision energies between 10-5 and 10 000 cm-1 using the NH3-He potential of Hodges and Wheatley [J. Chem. Phys. 114, 8836 (2001)]. State-to-state and total quenching cross sections from some selected initial states are presented. Rate coefficients for ortho-NH3 for the quenching 10+→00+ transition were obtained on potentials scaled to reproduce measurements of second virial coefficients with the results showing strong sensitivity to the potential, especially at low temperatures. Significant isotope effects are found in quenching cross sections in the cold to ultracold regime particularly in the region dominated by quasi-bound resonances, ~0.1 to 10 cm-1. As ammonia has been translationally cooled via Stark deceleration methods, it is a good candidate for experimental study of such effects at cold temperatures. Comparison of rate coefficients with available theoretical results are also presented.