The collision-induced, rototranslational absorption spectrum of compressed methane gas is computed, based purely on the reliably known, leading multipole-induced dipole components of CH4 molecular pairs. In contrast to previous work of the kind no ad hoc empirical corrections of unknown exchange force-induced dipole components are attempted. Not surprisingly, the calculated spectra show a sizeable absorption defect at virtually all frequencies, when compared to existing laboratory measurements. The defect suggests the presence of dipole-induction mechanisms in addition to those due to the leading multipole-induced dipole terms. The excess absorption, the differences between measured and calculated spectra, resembles in certain ways the excess absorption spectra seen at the same frequencies in methane-X gas mixtures, where X stands for helium, hydrogen, or nitrogen, respectively [ Buser and Frommhold J. Chem. Phys. 122 024301 (2005)]. To a large extent, the excess absorption seems to be related to collisional distortions of the tetrahedral frame of the unperturbed CH4 molecule.
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The rototranslational absorption spectrum of gaseous methane has been measured at seven different temperatures from 296 to 140 K. We have analyzed both the spectral moments and the experimental absorption shapes, assuming that only octupolar and hexadecapolar induction mechanisms contribute to the absorption. This assumption allows us to parameterize the temperature dependence of both the intensity and the shape of the absorption band. The results obtained indicate that other contributions to absorption are not negligible.
