Far infrared absorption in liquid methane: experiment and theory
Abstract
The far infrared absorption of liquid methane was measured in the frequency range 30- 550 cm- 1 at four temperatures from 91.2 K, near the freezing point, to 122.2 K. The zero, first and second spectral moments were obtained from these measurements and compared with theoretical values. Induction by the octopole and hexadecapole moments of methane was supplemented by an empirical anisotropic overlap induction mechanism. 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 with experimental moments improved. The theoretical spectral moments were used to evaluate parameters in an empirical translational line shape in the computation of the overall band shape of the far infrared spectrum. Because the band shape is dominated by the free rotational motion of methane, the exact functional form of the line shape function is not of paramount significance, except at very low frequencies and in the far high frequency wing. The discrepancy between the computed and measured spectra increases with decreasing temperature, with theory increasingly underestimating the experiment. Reasons for discrepancies between theory and experiment are presented.
- Publication:
-
Molecular Physics
- Pub Date:
- 1998
- DOI:
- 10.1080/00268979809482243
- Bibcode:
- 1998MolPh..93..573B