Semiclassical modeling of state-specific dissociation rates in diatomic gases

Abstract

A nonempirical, containing no adjustable parameters, theoretical model is suggested for calculations of state-specific dissociation rates in diatomic gases. Effects of molecular rotation and three dimensionality of collisions are consistently accounted for. The model is based upon a modified forced harmonic oscillator (FHO) scaling, with anharmonic frequency correction and energy symmetrization. The FHO scaling allows close-coupled calculations of multiquantum transitions between vibrational states, and it requires evaluation of collisional energy transfer to classical oscillator. Three-dimensional classical energy transfer models in both free-rotation and impulsive (sudden) approximations were used in conjunction with the FHO quantum scaling. The new theory describes the role of various degrees of freedom in dissociation both qualitatively and quantitatively. One of the predictions is that at low and moderate temperatures, dissociation is strongly preferential, with state-specific rates sharply increasing with vibrational energy; however, at high temperatures, the rate dependence on vibrational energy becomes less steep, turning into a virtually nonpreferential. Calculated thermal (equilibrium) and nonequilibrium dissociation rates of oxygen and nitrogen show a very good agreement with shock-tube experimental data taken from the literature.