Self- and air-broadened line shapes in the 2ν3 P and R branches of 12CH4

Abstract

In this paper we report line shape parameters of 12CH4 for several hundred 2ν3 transitions in the spectral regions 5891–5996cm−1 (P branch) and 6015–6115cm−1 (R branch). Air- and self-broadening coefficients were measured as a function of temperature; line mixing via off-diagonal relaxation matrix element coefficients was also obtained for 47 transition pairs. In total, nearly 1517 positions and intensities were retrieved, but many transitions were too weak for the line shape study. For this analysis, we used 25 high-resolution (0.0056 and 0.0067cm−1) and high signal-to-noise (S/N) spectra of high-purity 12CH4 and the same high-purity 12CH4 broadened by dry air recorded at different sample temperatures between 130K and 295K with the Bruker IFS 125HR Fourier transform spectrometer at JPL. Three different absorption cells were used (1) a White cell set to a path length of 13.09m for room temperature data, (2) a single-pass 0.2038m long coolable cell (for self-broadening) and (3) a multipass cell with 20.941m total path coolable Herriott cell (for air-broadening). In total there were 13 spectra with pure 12CH4 (0.27–599Torr) and 12 air-broadened spectra with total sample pressures of 80–805Torr and volume mixing ratios (VMR) of methane between 0.18 and 1.0. An interactive multispectrum nonlinear least-squares technique was employed to fit the individual P10–P1 and R0–R10 manifolds in all the spectra simultaneously. Results obtained from the present analysis are compared to other recent measurements.