Effective potential study of the rotational excitation of HD by collision with H2

Abstract

The effective potential formalism of Rabitz is extended to a general potential expressed in terms of relative or body‐fixed coordinates and applied to the study of the H2–HD rotationally inelastic collisions. The H2 and HD molecules are treated as rigid rotors and their interaction potential is derived from the H2–H2 potential. Long‐range quadrupole–dipole and quadrupole–quadrupole interactions are also considered. Quantum‐mechanical close‐coupling calculations for three‐dimensional collisions of para‐H2 and ortho‐H2 with HD are performed up to E=0.20 eV. The general features of the rotational excitation cross sections of HD are examined and their sensitivity to certain aspects of the potential are analyzed. In particular, the different roles of the short‐ and long‐range anisotropies are illustrated. It is found that the degree of anisotropy is meaningful only in relation to the magnitude of the ’’effectively’’ spherically symmetric part of potential. The rate constants for pure rotational transitions of HD are presented in the temperature range of 5 to 800 °K. The relation of these results to the quantitative interpretation of the thermal balance of interstellar clouds is pointed out. We have also calculated the rotational relaxation times for the lowest two levels of HD, which could be examined and compared with future sound absorption experiments to assess the accuracy of the H2–HD interaction potential.