$N_{2}$ AND Ar BROADENING AND LINE MIXING IN THE P AND R BRANCHES OF THE $\nu_{3}$ BAND OF METHANE

Abstract

$N_{2}$- and Ar-broadened spectra of the allowed P- and R-branch manifolds for J $\leq$ 10 in the $\nu_{3}$ band of $CH_{4}$ have been recorded from the Doppler limit to -67 kPa at T = 295 K using a tunable difference-frequency laser spectrometer. AS noted in our previous $study^{1}$ of the Q branch of $\nu_{3}$, the transition-dependent broadening coefficients are very similar for the different buffer gases, as are the P and Q branches, with the R branch exhibiting somewhat larger variations. Line intensities, pressure shifts and Dicke narrowing coefficients are also obtained. Strong line mixing among the blended tetrahedral components is observed, similar to that reported by Benner and $coworkers^{2}$,3 using FTIR measurements. An analysis is presented restricting line coupling among transitions of the same nuclear spin A, E or F symmetry with the sum of rhe mixing coefficients for a given symmetry in the J manifold constrained to zero.