The high-resolution visible overtone spectrum of CH4 and CD3H at 77 K

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Abstract
We have obtained visible overtone spectra with Doppler‐limited resolution of methane and trideuteromethane in the vicinity of six quanta of C–H stretch. At room temperature, the methane spectrum is unresolved. Upon cooling to 77 K in a specially designed photoacoustic cell, methane shows a complicated but rotationally resolved spectrum. The widths of all features in the spectrum are consistent with Doppler broadened linewidths at 77 K. Efforts to assign this spectrum are in progress. The overtone spectrum of CD3H has been recently studied by other workers at a resolution of 0.5 cm−1 [Perry, Moll, Kupperman, and Zewail (preprint)]. The spectrum in this region consists of two parallel bands, one at 16 156 cm−1 and another at 16 230 cm−1. These are assigned as arising from a Fermi resonance between the pure C–H overtone 6ν1 and a combination with the degenerate C–H bend, 5ν1+2ν5. A high resolution spectrum taken at 77 K shows nearly completely resolved K‐subband structure for both bands. The rotational constants of both states cannot be rationalized purely on the basis of the above vibrational assignment, although it is the most reasonable explanation of the observed vibrational spectrum. Our rotational analysis of the bands indicates that both of these states are in Coriolis resonance with another state or states which are not seen in our spectrum.