Multispectrum analysis of 12CH 4 from 4100 to 4635 cm −1: II. Air-broadening coefficients (widths and shifts)

Abstract

Room temperature Lorentz air-broadened halfwidth and pressure-induced air-shift coefficients were measured for 1011 transitions in the octad region of methane between 4100 and 4635 cm −1. These measurements were made by analyzing 10 laboratory absorption spectra recorded at 0.011 cm −1 resolution using the McMath–Pierce Fourier transform spectrometer located at the National Solar Observatory on Kitt Peak, Arizona. The spectra were obtained using two absorption cells with path lengths of 2.05 and 150 cm. The total sample pressures ranged from 99 to 400 torr with CH 4 volume mixing ratios of 0.01 in dry air. The spectral line parameters were retrieved using a multispectrum nonlinear least squares technique. Transitions belonging to five bands of the octad polyad were observed, namely ν 2 + 2 ν 4, ν 1 + ν 4, ν 3 + ν 4, 2 ν 2 + ν 4, and ν 2 + ν 3. The numbers of measurements by bands are: 33 for ν 2 + 2 ν 4, 180 for ν 1 + ν 4, 635 for ν 3 + ν 4, 33 for 2 ν 2 + ν 4, and 130 for ν 2 + ν 3. Transitions with rotational quantum number J up to 16 are included in the analysis. The measured width and shift coefficients vary according to the tetrahedral symmetry species and the rotational quantum numbers of the transitions. The retrieved parameters from this study are compared with prior results, in part to estimate absolute accuracy and determine the extent of vibrational dependence of widths and shifts.