Theory of the Rare-Gas-Induced Q Branch in HCl Vibration–Rotation Spectra at High Pressures

A theory for the rare‐gas‐induced $Q$ branch in the HCl fundamental vibration–rotation band at high pressures is developed. The interaction responsible for the phenomenon arises from the displacement of the center of attraction (repulsion) to the rare‐gas atom from the center of mass in HCl. The short‐range behavior of this interaction is studied, and it is concluded that the displacements between centers in short‐ and long‐range interactions are equal. Based on this conclusion, the absorption intensity distributions for HCl–Ar and HCl–He mixtures at 400 amagat, 195°K are computed, as well as that for the HCl–Ar mixture at 450 amagat, 300°K. Certain general aspects of the calculations reveal the reasons behind various observed characteristics of Q‐branch induction, while the results of explicit calculations of absorption intensity contours show substantial quantitative agreement with experiment. Among other things the over‐all magnitude of the $Q$ branch, its dependence on rare‐gas species, and its dependence on temperature are successfully accounted for in the present analysis. The principal discrepancies arise from underestimation of the widths of the various components of the absorption intensity distributions.