Comprehensive Theory for the Broadening, Shifting and Narrowing of Hydrogen-Fluoride and Hydrochloric Acid Fundamental Band Absorption Profiles.

A comprehensive theory for the broadening, shifting and narrowing of molecular radiative profiles at optical wavelengths is developed within the impact approximation and applied to nitrogen broadening of HF and HCl fundamental band transitions. The results of the semiclassical calculation of the broadening, shifting and narrowing parameters are in good agreement with the experimental results of Pine and Looney. The present calculations show the observed narrowing at low rotational quantum numbers to be attributable to the speed dependence of the broadening rate. At intermediate and high rotational quantum numbers, there is a reduction to the narrowing from speed dependence of the broadening rate in both the HF and HCl systems and the effects of velocity-changing collisions become manifest. In the HF -N_2 system the observed narrowing at J >=q 7 is due solely to velocity change effects, whereas in the HCl-N_2 system the high J transitions have contributions from both the speed dependence of the broadening rate and from velocity -changing collisions. The widths and shifts of the fundamental band transitions of HCl and HF under self and N_2 pressure are calculated using the resolvent operator formalism of Kolb-Griem-Baranger. Time-development operator matrix elements are evaluated in a manner that is similar to the linked-cluster techniques, except that all anisotropic terms that are bilinear and second order are accounted for, as well as isotropic effects to all orders. The calculations include the use of a parabolic trajectory model and explicit velocity averaging. It is found that the major contributions to the linewidths arise from dipole-quadrupole, quadrupole -quadrupole and vibrationally dependent isotropic dispersion forces. Due to the reduction of the dipole-quadrupole and quadrupole-quadrupole contributions with increasing |m|, the widths decrease with a leveling off at high |m|, where dispersion forces dominate. A comparison between calculated and measured widths and shifts, for a range of temperatures from T = 202-295^circK, shows good overall agreement.