Multispectrum analysis of CH4 from 4100 to 4635 cm: 1. Self-broadening coefficients (widths and shifts)

The Lorentz self-broadening (halfwidths) and self-induced pressure-shift coefficients were measured for the first time in the octad region of methane. All spectra were recorded at 0.011 cm⁻¹ resolution using the McMath-Pierce Fourier transform spectrometer located at the National Solar Observatory on Kitt Peak, Arizona. ¹²C-enriched CH₄ gas samples at room temperature were used, and high signal-to-noise ratios of 2000:1 were achieved. The multispectrum nonlinear least squares fitting technique enabled us to fit simultaneously a total of 10 high-resolution laboratory absorption spectra consisting of six self-broadened and four air-broadened spectra. In this paper, we report the self-broadened widths and self-induced pressure shift coefficients for 1423 transitions belonging to five bands with a maximum J of 16. The numbers of measurements by bands are: 71 for ν₂ + 2ν₄, 202 for ν₁ + ν₄, 824 for ν₃ + ν₄, 58 for 2ν₂ + ν₄, and 268 for ν₂ + ν₃. The observed widths varied from 0.045 to about 0.090 cm⁻¹ atm⁻¹ at 296 K. The measured pressure-shift coefficients had values extending from about −0.020 to −0.005 cm⁻¹ atm⁻¹ at 298.3 ± 1.2 K. The results obtained for the broadening coefficients in the various bands were compared with each other and with measurements reported in the literature for other methane bands.