The 2ν4 isotropic and anisotropic Raman bands of CH4

Abstract

AbstractThe Raman spectrum of the very weak 2ν4 band of CH4 has been recorded in the region 2460–2675 cm−1 with a spectral resolution of about 0.30 cm−1 and with two different orientations of the linearly polarized exciting light relative to the direction of observation. The two exposures are used to obtain the purely isotropic and the purely anisotropic Raman spectrum of the band. The two spectra are interpreted in terms of a model which takes explicitly into account vibrational and rotation‐vibrational interactions with other vibrational states. Using molecular constants determined primarily from infrared spectroscopy, theoretical contours are calculated which agree very satisfactorily with the experimental ones. For the isotropic spectrum it is found that two different values of the ratio between the 2ν4(A1) and the ν1 transition moment matrix elements reproduce the spectrum equally well. It is shown that the major part of the intensity in the isotropic and anisotropic Raman spectrum is borrowed from the ν1(A1) and the ν3(F2) fundamentals, respectively, through Fermi‐type interactions, and it is also shown that the 2ν4 isotropic Raman band would have been 4–6 times more intense, if the 2ν4(A1) transition moment matrix element had been negligible.