Knowledge Bank

Self- 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 $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}$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$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$) 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.