Cavity ring down absorption at low temperatures: C–H spectra (Δυ = 1–6) of CH3D and C–H overtones (Δυ = 5, 6) of CH2D2 and CH4

Abstract

The spectra of the C–H stretch fundamental and overtones (Δυ = 1–6) of CH3D have been recorded. Absorptions in the visible were obtained with a phase shift cavity ring down technique where an optical cavity is inside a low temperature cryostat. Absorptions below 12,000 cm−1 were observed with a Fourier transform spectrophotometer. The local mode harmonic frequency and anharmonicity were obtained and used with the harmonically coupled anharmonic oscillator (HCAO) model to calculate energy levels and assign the absorption bands to particular transitions. Overtone absorptions (Δυ = 5 and 6) of CH4 and CH2D2 have also been obtained. The integrated absorption was calculated as a function of the density of the gas samples and used to obtain the band strength and the cross- section of the Δυ = 5 and 6 C–H transitions for each molecule. Cross–sections for CH4 agree within 10% with traditional absorption measurements with a multiple pass cell at high pressures. The importance of the new experimental technique is emphasized for laboratory studies of planetary atmospheres.