Existing rototranslational collision-induced absorption (CIA) spectra of methane pairs are analyzed with the help of spectral profiles computed from quantum mechanics. Dipoles induced by octopolar and hexadecapolar fields, hexadecapolar overlap, and the gradient of the octopolar field are considered. The spectral contributions of both bound and free pairs of molecules are accounted for. The analysis which suggests a centrifugal distortion of rotating methane molecules permits one to reproduce from theory the measured CIA spectra at all temperatures (126–300 K) and over the full range of frequencies (> 700 cm−1 at high temperatures) with rms deviations that are smaller than the experimental uncertainties. The values of the octopole and hexadecapole moments of (nonrotating) CH4 molecules needed for that purpose are consistent with state-of-the-art ab initio computations for the first time in such work.
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