Air-, N 2-, and O 2-broadening and shift coefficients in the ν 3 spectral region of 12CH 4

Abstract

Pressure-broadening and pressure-induced shift coefficients have been measured for more than 450 transitions in the ν 3 region of 12CH 4, between 2650 and 3200 cm -1. The results were obtained by analyzing 29 room-temperature laboratory absorption spectra recorded with 0.01-cm -1 resolution using the McMath Fourier transform spectrometer of the National Solar Observatory (NSO). Absorption cells with path lengths of 5, 25, and 150 cm, sample pressures of 60–550 torr, and CH 4 volume mixing ratios of 0.01 or less in dry air, N 2 or O 2 were used. The broadening and shift coefficients were determined using a nonlinear least-squares spectrum fitting technique. Absorption lines belonging to ν 3, ν 2+ν 4, 2ν 2 2 bands of 12CH 4 were observed in the spectra. Transitions up to J̋ = 17 in the ν 3, J̋ = 12 in the ν 2+ν 4, J̋ = 9 in the 2ν 2 4 and J̋ = 4 in the 2ν 2 2 bands were included in the analysis. Variations of the measured parameters with tetrahedral symmetry species and rotational quantum numbers of the transitions involved are discussed. Comparisons of the retrieved parameters in various bands in both 12CH 4 and 13CH 4 are performed in order to determine the extent of vibrational dependence.