Line-shape analysis of speed-dependent collisional width inhomogeneities in CO broadened by Xe,N2, and He

We present high-resolution (${10}^{\mathrm{-}4}$ ${\mathrm{cm}}^{\mathrm{-}1}$) and high signal-to-noise ratio (2000) infrared line shapes of CO perturbed by Xe, ${\mathrm{N}}_2$, and He in the intermediate regime from Doppler to collision broadening. It is found that the pressure-broadened component of the line shape cannot be fit by a Lorentzian line shape, and the largest deviation occurs in the core of the line. A speed-dependent collisional width is required to fit the spectra broadened by the larger Xe perturber, for which the collision rate of a CO molecule is determined primarily by its own speed. A line-shape model based on the theory of Robert et al. [Phys. Rev. A 47, R771 (1993)], folding in the effect of Dicke narrowing, is derived. The speed-dependent mechanism also appears to explain the measurements made by Duggan et al. [Phys. Rev. A 48, 2077 (1993)] in a dilute CO:${\mathrm{N}}_2$ mixture, in which a non-Lorentzian collision broadening scheme was suggested.