Line Shape and Collision Effects in the Microwave Wing of Far‐Infrared Rotational Lines

Abstract

The microwave absorption very far in the low‐frequency wing of the pure rotational spectra of HCl, DCl, and HBr has been measured in order to investigate the applicability of pressure‐broadening theory in the region many linewidths removed from the resonant frequencies. Data for mixtures of HCl and HBr with the foreign gases He, H₂, Ar, N₂, CH₄, C₂H₆, CO, SF₆, and CO₂ are also reported. Experimental results are compared with the Van Vleck—Weisskopf equation, which is used to indicate an upper limit of the absorption. Data for the mixtures with He and H₂ are in accord with this line shape. Data for the remaining mixtures and the pure gases are not in agreement, the observed absorption being from 2 to 8 times greater than predicted. These results suggest the presence of an additional absorption region which is centered at a lower frequency than the normal rotational spectrum and which may be attributed to interacting molecular pairs.