Temperature dependence of the collisional interference in the pure rotational far-infrared spectrum of HD

The far-infrared pure rotational absorption spectrum of HD and HD-X mixtures was measured at 77 and 195 K. Specifically, the rotational lines R(0)--R(2) were observed for HD-HD, HD-He, HD-Ne, and HD-H2 at 77 K and R(0)--R(3) for HD-HD, HD-He, and HD-Ar at 195 K. Refined values of the permanent dipole moment of HD are deduced. Collisional interference in the spectrum is characterized by a parameter proportional to the ratio of the average-induced dipole moment to the permanent dipole moment. Its variation with temperature, particularly a change in sign, is not explainable by current theory. The spectral line-shape parameters of width, frequency shift, and asymmetry are determined and their density and temperature behavior discussed with reference to the impact theory of line broadening