Far-infrared rotational spectrum of HD: Line shape, dipole moment, and collisional interference

The rotational transitions R(0) to R(3) in pure HD gas have been studied in absorption with 0.06-cm/sup -1/ resolution at densities from 2 to 68 amagat at 295 K. The permanent dipole moment of HD was found to be 8.83(28), 7.94(2), 7.88(3), and 8.43(10) for R(0) to R(3), respectively, in units of 10/sup -4/ D. These are in general agreement with ab initio calculations except for an unpredicted strong rotational state dependence. Collisional interference was observed and the interference parameter, proportional to the ratio of the average induced dipole moment to the allowed moment, is negative for R(0) and R(1), positive for R(2) and R(3), and of a magnitude consistent with calculation. Comparisons are made with earlier work, particularly that of McKellar [Can. J. Phys. 64, 227 (1986)]