Multispectrum analysis of 12CH 4 in the ν4 band: I. : Air-broadened half widths, pressure-induced shifts, temperature dependences and line mixing

Abstract

Lorentz air-broadened half widths, pressure-induced shifts and their temperature dependences have been measured for over 430 transitions (allowed and forbidden) in the ν 4 band of 12CH 4 over the temperature range 210–314 K. A multispectrum non linear least squares fitting technique was used to simultaneously fit a large number of high-resolution (0.006–0.01 cm −1) absorption spectra of pure methane and mixtures of methane diluted with dry air. Line mixing was detected for pairs of A-, E-, and F-species transitions in the P- and R-branch manifolds and quantified using the off-diagonal relaxation matrix elements formalism. The measured parameters are compared to air- and N 2-broadened values reported in the literature for the ν 4 and other bands. The dependence of the various spectral line parameters upon the tetrahedral symmetry species and rotational quantum numbers of the transitions is discussed. All data used in the present work were recorded using the McMath–Pierce Fourier transform spectrometer located at the National Solar Observatory on Kitt Peak.