Experimental and theoretical studies of absorption in microwindows of the ν₄-band of methane and methane-hydrogen

The absorption coefficients of room temperature CH₄ and CH₄-H₂ mixtures were measured in the 1300-1360 cm^-1 region of v₄ band at about 295 K as a function of pressure. These results were analyzed at frequencies in the most transparent regions of various microwindows in order to study the far wings of self- and H₂-broadened lines in the R-branch of the ν₄ band. The experimental results were compared with synthetic spectra computed from the addition of Lorentzian line contributions. From such comparisons it appears that the high-frequency wings of the lines of the Q-branch decay more rapidly than do Lorentz wings. At higher wave numbers, the measured absorption tends to be equal and then less than the computed Lorentz absorption, although these deviations may not be significant in view of the uncertainties of the measurements and line parameters used in the computed spectra. Nonetheless, the computed absorption when including line coupling is in better overall agreement with the experiment and confirms the role of the Q-branch in producing the sub-Lorentzian absorption noted above.