COLLISION-INDUCED ABSORPTION IN MIXTURES OF SYMMETRICAL LINEAR AND TETRAHEDRAL MOLECULES - METHANE NITROGEN

The far infrared absorption of a CH4-N2 mixture was measured at 297, 1 95, and 162 K from 30 to 650 cm-1. The spectral invariants gamma1 and alpha1, proportional, respectively, to the zeroth and first spectral m oments, 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 symm etrical linear molecule includes contributions not previously consider ed. Whereas the theoretical values of gamma1 are only somewhat greater than experiment at all temperatures, including those obtained from th e data of Dagg et al, the theoretical values of alpha1 are significant ly lower than the experimental values. From the theoretical spectral m oments for the various induced dipole components, the parameters of th e BC shape were computed and theoretical spectra were constructed. Goo d agreement was obtained at the lower frequencies, but with increasing frequencies the theoretical spectra were increasingly less intense th an 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 i n alpha1) can be removed by any conceivable modification in the potent ial.