SPECTRAL LINE SHAPES IN THE ν3 Q BRANCH OF 12CH4 NEAR 3.3 μm

Detailed knowledge of spectroscopic parameters for prominent Q branches of methane is necessary _x000d_ for interpretation and modeling of high resolution infrared spectra of terrestrial and planetary atmospheres. _x000d_ We have measured air-broadened line shape parameters in the Q branch of $^{12}$CH$_4$ in the $nu_3$ fundamental _x000d_ band for a large number of transitions in the 3000 to 3023 cm$^{-1}$ region by analyzing 13 room-temperature laboratory absorption spectra. Twelve of these spectra were recorded with 0.01 cm$^{-1}$ resolution using the McMath-Pierce _x000d_ Fourier transform spectrometer (FTS) of the National Solar Observatory (NSO) on Kitt Peak, and one _x000d_ higher-resolution ($sim$0.0011 cm$^{-1}$) low pressure ($sim$1 Torr) spectrum of methane was obtained using the _x000d_ Bruker IFS 120HR FTS at the Pacific Northwest National Laboratory (PNNL) in Richland, WA. _x000d_ The air-broadened spectra were recorded using various absorption cells with path lengths of 5, 20, 25, and 150 cm, _x000d_ total sample pressures between 50 and 500 Torr, and CH$_4$ volume mixing ratios of 0.01 or less. _x000d_ All 13 spectra were fit simultaneously covering the 3000-3023 cm$^{-1}$ spectral region using a multispectrum nonlinear least squares techniquefootnote{D.~C.~Benner, C.~P.~Rinsland, V.~Malathy Devi, M.~A.~H.~Smith, D.~Atkins, textit{JQSRT} _x000d_ textbf{53} (1995) 705-721.} to retrieve accurate line positions, absolute intensities, Lorentz air-broadened widths _x000d_ and pressure-shift coefficients. Line mixing using the off-diagonal relaxation matrix element _x000d_ formalismfootnote{A.~Levy, N.~Lacome, C.~Chackerian, Collisional line mixing, in textit{Spectroscopy of _x000d_ the Earth$^prime$s Atmosphere and Interstellar Medium}, Academic Press, Inc., Boston (1992) 261-337.} was measured _x000d_ for a number of pairs of transitions for the CH$_4$-air collisional system. The results will be _x000d_ compared to values reported in the literature._x000d