Near-infrared and visible spectroscopy of CH3D in liquid argon solutions

The near-infrared and visible spectra of CH3D in liquid argon solutions have been measured around 95 K. The fundamental and overtones, Delta upsilon = 1, 2, 3, 4, and 5 around the C-H stretch and Delta upsilon = 1, 2, and 3 of the C-D stretch, are reported. Combination bands between 3500 and 5000 cm(-1) (Delta upsilon = 3/2) and between 6500 and 7600 cm(-1) (Delta upsilon = 5/2) of the C-H stretching vibration were also measured. Measurements were made using a cryostat, a low-temperature cell, and a Fourier transform IR and near-IR spectrometer. Visible spectra were recorded with a photoacoustic spectrometer employing resonant continuous wave laser excitation and piezoelectric detection. Spectra in solutions are greatly simplified compared to the gas phase. This simplification is attributed to a narrowing of the rotational distribution at low temperatures and partial hindering of the rotational motion of sample molecules. Peak positions are systematically red-shifted with respect to the gas phase, and the magnitude of the shift increases with the vibrational quantum number. Numerous transitions unobserved in the gas phase carry oscillator strength in solution; thus, significant intensity redistribution occurs in the solution environment. The harmonic frequency and anharmonicity were obtained from a Birge-Sponer fit of the C-H vibrations, and the interbond coupling parameter was obtained from the observed splitting of the fundamental C-H stretch. These three parameters were used with a harmonically coupled anharmonic oscillator model to calculate frequencies and assign absorption bands.