Raman spectrum of β-CH4 in the intramolecular and lattice regions

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Abstract
The Raman spectrum of β‐CH4, or phase II of solid methane, has been observed in the intramolecular and lattice regions. The ν4 mode and the combination bands 2ν4 and ν2+ν4 have been observed for the first time. The spectrum of the 2ν4 band provides further evidence of free rotation in β‐CH4. However, the rotation–vibration spectrum of Oh molecules was not observed in the ν4 band due to its low intensity. In the lattice region, three first‐order Raman lines were observed, as predicted by theory. A broad, asymmetric line in the lattice region, identified as a librational mode, gives direct evidence for the existence of a group of excited librational states about 50 cm−1 from the librational ground state of D2d molecules. The measured temperature dependence of the frequency and width of this line is compared to previous theoretical calculations.