Knowledge Bank

SELF-, $N2-$, $O2-$, $H2-$, $Ar-$ and He-broadening coefficients, pressure shifts and intergrated intensities have been measured for most transitions in the Q branch of the $\nu 3$ fundamental band of methane using a difference-frequency laser spectrometer. A systematic dependence of the broadening coefficients on the tetrahedral symmetry species and order index is observed, with striking similarities for $N2$, $O2$, and Ar and for $H2$ and He buffer gases. Comparison with earlier measurements on the bands and branches of methane indicates very little vibrational, branch or carbon isotope dependence. Dicke narrowing is evident at intermediate pressures, yielding an average narrowing coefficient and an optical diffusion constant for each gas mixture. A small amount of line mixing is evident for strongly overlapped lines near atmospheric pressure from nonlinear deviations of the observed spectra from the contours extrapolated from lower pressure measurements.