Collision-induced absorption in mixtures of symmetrical linear and tetrahedral molecules: Methane-nitrogen

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The far infrared absorption of a CH4-N2 mixture was measured at 297, 195, and 162 K from 30 to 650 cm-1. The spectral invariants γ1 and α1, proportional, respectively, to the zeroth and first spectral moments, due to bimolecular collisions between CH4 and N2 were obtained from these data and compared with theoretical values. The theory for collision-induced dipoles between a tetrahedral and a diatomic or symmetrical linear molecule includes contributions not previously considered. Whereas the theoretical values of γ1 are only somewhat greater than experiment at all temperatures, including those obtained from the data of Dagg et al., the theoretical values of α1 are significantly lower than the experimental values. From the theoretical spectral moments for the various induced dipole components, the parameters of the BC shape were computed and theoretical spectra were constructed. Good agreement was obtained at the lower frequencies, but with increasing frequencies the theoretical spectra were increasingly less intense than the experimental spectra. Although the accuracy of the theoretical results may suffer from the lack of a reliable potential function, it does not appear that this high frequency discrepancy (also reflected in α1) can be removed by any conceivable modification in the potential. © 1993 American Institute of Physics.

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The Journal of Chemical Physics