Infrared fundamental bandshape of HCl in fluid SF6: A modified-rotor description

Abstract

The fundamental bandshape of HCl diluted in fluid SF₆ has been studied in the temperature range 283–340 K at two constant densities of the solvent. It is found that the previously developed quasi-free rotor theory realistically reproduces the experimental band area-normalized intensity distribution in the band wings, but overestimates intensity near the maxima of the P- and R-branches. A concept of a potential barrier hindering rotation of the solute molecules is introduced and a new modified-rotor version of the spectral theory is formulated, which takes into account depopulation of the lower-energy rotational states. We show that the modified-rotor theory significantly improves agreement with the experiment. The rotationally-hindered contribution to the bandshape is separated from the measured spectral profiles and its temperature and density dependence is discussed.