Interdependence of Spectral Line Breadth, Sound Velocity Dispersion, and Viscosity of Small Molecules in the Gas Phase

Abstract

An examination of the data on bimolecular collisions of small molecules has produced several new insights and predictions. The data on viscosity, sound velocity dispersion, and spectral line breadths in the microwave, infrared, and Raman regions have been assembled for H2, O2, N2, CH3Cl, and HCN. These data have been put on an equivalent basis of calculated or observed spectral line breadth constants. When this is done, the relative importance of elastic and inelastic rotational collisions may be assessed. The data on H2 are here used to predict a collisional narrowing of the ordinary Doppler linewidth; this narrowing effect can be found in the literature, where the significance of the extremely narrow lines has not been fully appreciated. The line breadths observed in the microwave spectrum of oxygen are seen to be peculiar to some phenomenon which reorients electron spins but does not alter rotational states. Spin exchange is here identified as the most probably mechanism for microwave line broadening in O2. The differences between isotropic and anisotropic linewidths in the Raman spectra of H2, O2, and N2 indicate that a thorough theoretical treatment of Raman linewidths is required. Some additional experiments on line breadths in the Raman spectra of H2, O2, and N2 are suggested. The basic approximation in Anderson's impact theory for broadening of spectral absorption and emission lines is seen to be quantitatively invalid for self-broadening of these homonuclear diatomic molecules. This is the approximation of classical (straight) paths. Two new phenomena are predicted for the propagation of rotational energy in small polar molecules. The first of these is a separation of the rotational sound velocity dispersion into two regions for symmetrictop molecules. The region of lower frequency dispersion should account for R/2 of heat capacity, and be associated with changes in rotation about the unique axis of the molecule. Small polar molecules with large dipoles (particularly the alkali halide vapors) should also exhibit a peculiar form of energy propagation in the rotational degrees of freedom only. This ``roton propagation'' may also influence the behavior of shock waves in polar gases and the heat conductivity at very low pressures.