Vibration—Rotation Spectra of CH4 and CD4 Impurities in Xenon, Krypton, and Argon Crystals

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Abstract
The infrared spectra of CH4 and CD4, present as substitutional impurities in crystals of argon, krypton, and xenon, were studied at temperatures ranging from 5° to 40°K. Both v3 and v4 of CH4 in xenon showed a simple four line pattern which is consistent with that expected for a slightly hindered rotor. In krypton and argon a fifth line appeared on v3, as expected from King's theoretical calculations for a tetrahedral rotor in an octahedral field. In addition, argon showed absorption due to pairs or higher aggregates of CH4 molecules. The rotational spectrum of CD4 is much more highly perturbed. Detailed assignments to hindered rotational levels could not be made but the general features are in accord with King's model. The frequencies of the CH4 and CD4 absorption bands increase steadily in going from xenon to krypton to argon. This can be accounted for by the decrease in the sizes of the cavities occupied, with corresponding larger repulsive interactions between the CH4 or CD4 and the host atoms.