Multispectrum analysis of 12CH4 in the ν4 spectral region: II. Self-broadened half widths, pressure-induced shifts, temperature dependences and line mixing

Abstract:

Accurate values for line positions, absolute line intensities, self-broadened half widthand self-pressure-induced shift coefficients have been measured for over 400 allowedand forbidden transitions in the n4 band ofmethane (12CH4). Temperature dependencesof half width and pressure-induced shift coefficients were also determined for many ofthese transitions. The spectra used in this study were recorded at temperatures between210 and 314K using the National Solar Observatory’s 1m Fourier transform spectro-meter at the McMath-Pierce solar telescope. The complete data set included 60 high-resolution (0.006–0.01cm1) absorption spectra of pure methane and methane mixedwith dry air. The analysis was performed using amultispectrum nonlinear least squarescurve fitting technique where a number of spectra (20 or more) were fit simultaneouslyin spectral intervals 5–15cm1 wide. In addition to the line broadening and shiftparameters, line mixing coefficients (using the off-diagonal relaxation matrix elementformalism) were determined for more than 50 A-, E-, and F-species transition pairs in Jmanifolds of the P- and R-branches. The measured self-broadened half width and self-shift coefficients, their temperature dependences and the line mixing parameters arecompared to self-broadening results available in the literature and to air-broadenedparameters determined for these transitions from the same set of spectra.