Collisional interference in the foreign-gas-perturbed far-infrared rotational spectrum of HD

The rotational transitions R(1), R(2), and R(3) of HD have been studied in absorption with 0.06-cm/sup -1/ resolution in mixtures of HD with He, Ne, Ar, Kr, Xe, H2, and N2 at 295 K and at densities between 3 and 62 amagat. Collisional interference was observed and the interference parameter, proportional to the ratio of the average induced moment to the allowed moment, showed a dependence on system size and ranged from +10 x 10/sup -3/ amagat/sup -1/ for light perturbers to -24 x 10/sup -3/ amagat/sup -1/ for the heavier ones. These interference effects in the pure rotational band are an order of magnitude smaller than those of the fundamental vibrational band and show a strong J dependence not predicted within current theory. The sign of the isotropic overlap component of the induced moment was determined experimentally in an unambiguous way and its trend was found to be within theoretical expectation; specifically, the sign changes as the mass increases. The line shapes were mostly symmetrical and Lorentzian; asymmetry was observed mainly for the R(1) transition with the low-frequency side of the lines super-Lorentzian and the high-frequency side sub-Lorentzian. Investigation of the effect of collision duration on the autocorrelation function of the allowed moment showed it was not important to the integrated allowed absorption