A multispectrum analysis of the ν4 band of 13CH4: Widths, shifts, and line mixing coefficients

Line positions, Lorentz air-broadened half width and air pressure-induced shift coefficients have been measured for nearly 200 transitions in the ν4 band of 13CH4 from high-resolution spectra recorded with the McMath-Pierce Fourier transform spectrometer. Three room temperature spectra of 13CH4 used in the previous study of Malathy Devi et al. (Air-broadened Lorentz halfwidths and pressure-induced line shifts in the ν4 band of 13CH4. Appl. Opt. 1988; 27: 2296-2308) were analyzed together with a large number of additional spectra of self- and air-broadened CH4 recorded at 210-314 K and one room-temperature spectrum of self-broadened 13CH4. Analyses applying the multispectrum nonlinear least squares fitting technique were performed to retrieve the spectral line parameters. In addition to air-broadened half width and shift coefficients, self-broadened half width and shift coefficients were determined for at least 56 13CH4 ν4 transitions. Off-diagonal relaxation matrix element coefficients for air-broadened line mixing were also determined for 28 pairs of P and R transitions in a number of J manifolds, and mixing parameters for self-broadening were also determined for some of these pairs. Temperature-dependences of the pressure-induced shift and mixing parameters for the 13CH4 lines could not be determined from the spectra used in the present analysis, but temperature dependences of the half width coefficients were determined for the strongest transitions. The results of this study are compared with other studies of air- and self-broadened 13CH4 and 12CH4.

Research Highlights►Laboratory FTS spectra analyzed by multispectrum fitting to determine air- and self-broadening parameters of 13CH4. ►Temperature-dependence of 13CH4 half widths from 210 to 314 K. ►Higher accuracy achieved than previous studies of 13CH4. ►Line mixing for 13CH4 transitions determined for the first time.