Theory and Measurement of Pressure-Induced Shifts of HCl Lines Due to Noble Gases

Pressure shifts of HCl absorption lines in both the 1–0 and 2–0 bands caused by five noble gases have been measured. An outstanding feature of these measurements is that the shifts are not the same for all lines of a band, but there is a strong dependence on the rotational quantum number J. However, there is evidence that the shifts tend to a constant limit for high J numbers. In order to interpret these results induction- and dispersion-type van der Waals forces were considered and used in conjunction with existing line-shape theories. In cases where perturbation theory can be applied, the limit shifts were accounted for fairly well, although no satisfactory explanation has emerged for the J dependence. Attention is drawn to the significance of observations of this sort as a means of obtaining information about intermolecular forces and molecular constants. Of particular interest is the possibility of obtaining such information for molecules in specific quantum states.