Quantised rotation of HBr in liquid SF6

Absorption data on HBr in liquid SF₆ at 296 K are presented in the frequency range 20–200 cm^?1. The broadened J → J + 1 transitions are clearly resolved, more so at the higher frequencies. Although noisy, the data are good enough to discriminate between two models of liquid phase rotational dynamics. It is found that the modified M and J diffusion model of Frenkel and Wegdam rests on sounder hypotheses than a formalism, based on a truncated Mori expansion, that does not predict preferential broadening of the lower frequency J → J + 1 lines.