FAR-INFRARED ABSORPTION IN LIQUID METHANE - EXPERIMENT AND THEORY

The far infrared absorption of liquid methane was measured in the freq uency range 30-550 cm(-1) at four temperatures from 91.2 K, near the f reezing point, to 122.2 K. The zero, first and second spectral moments were obtained from these measurements and compared with theoretical v alues. Induction by the octopole and hexadecapole moments of methane w as supplemented by an empirical anisotropic overlap induction mechanis m. Agreement between theoretical and experimental spectral moments was generally in the range of 10-20%. The computed spectral moments were corrected for quantum effects; with these corrections the agreement wi th experimental moments improved. The theoretical spectral moments wer e used to evaluate parameters in an empirical translational line shape in the computation of the overall band shape of the far infrared spec trum. Because the band shape is dominated by the free rotational motio n of methane, the exact functional form of the line shape function is not of paramount significance, except at very low frequencies and in t he far high frequency wing. The discrepancy between the computed and m easured spectra increases with decreasing temperature, with theory inc reasingly underestimating the experiment. Reasons for discrepancies be tween theory and experiment are presented.