Vibrational overtone spectroscopy of methane in liquid argon solutions

Abstract

The spectra of the fundamental, and first and second overtones (Δ ν = 1,2,3) around the C-H stretch of methane in liquid argon solutions have been measured at approximately 95 K. Combination bands involving one quantum of a bending mode in the region 3500–5000 cm -1 and three quanta of bending modes in the region 6500–7600 cm -1 were also measured. The near-infrared (IR) spectra of pure liquid methane was recorded between 6500 and 12 000 cm -1 at 94 K. Measurements were made using a cryostat, a low-temperature cell, and a Fourier transform IR and near-IR spectrometer. Visible spectra of the fourth (Δ ν = 5) and fifth (Δ ν = 6) overtones of liquid methane and methane in liquid argon solutions were measured at 95 K. Visible spectra were recorded with a photoacoustic spectrometer employing resonant continuous-wave excitation and piezoelectric detection. Spectra in solutions are greatly simplified compared with 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. The harmonic frequency and anharmonicity were obtained from a Birge–Sponer fit of the C–H vibrations. Previously reported assignments in normal-mode notation and local-mode calculations of energy levels of methane were used to assign the vibrational transitions observed in our experiments. The local mode notation is based on the harmonically coupled anharmonic oscillator model. Linewidths of the pure overtones increase proportionally as a function of the vibrational quantum number ν. There is indication that, for transitions with Δ ν = 5 and 6, the bandwidths are due to a dephasing mechanism in the slow modulation limit.