MULTISPECTRUM ANALYSIS OF THE $\nu_4$ AND $\nu_2$ BANDS OF $^{12}$CH$_4$

Author Institution: Science Directorate, NASA Langley Research Center, MS 401A, Hampton, VA 23681-2199; Department of Physics, The College of William and Mary, Williamsburg, VA 23187-8795Self- and air-broadened halfwidth and pressure-induced shift coefficients and their temperature dependences have been determined for a number of transitions in the $\nu_4$ and $\nu_2$ bands of $^{12}$CH$_4$ from laboratory absorption spectra recorded at room temperature and below with the McMath-Pierce Fourier transform spectrometer of the National Solar Observatory. In addition, accurate line center positions and absolute intensities were determined. The results were obtained by using a multispectrum nonlinear least squares technique} \underline{\textbf{53}}, 705-721 (1995).} to fit simultaneously 20 or more spectra recorded at high resolution (0.006-0.01 cm$^{-1}$) with path lengths between $\sim$ 1 and 150 cm and sample temperatures ranging from 210 to 314 K. For both self- and air broadening in the J-manifolds of the P and R branches of the $\nu_4$ band, we observed line mixing between certain transitions within the same manifold. In these cases the off-diagonal relaxation matrix elements were determined in the fits. Most of the air-broadened widths of unmixed $\nu_4$ lines retrieved using the multispectrum fits agree well with earlier values determined from single-spectrum fits,} \underline{\textbf{48A}}, 1257-1272 (1992).} and the multispectrum results have smaller statistical uncertainties. The inclusion of line mixing in the fits was seen to have a greater effect on the retrieved values of the line shifts than on the retrieved values of other parameters