Semiclassical modeling of state-specific dissociation rates in diatomic gases

A nonempirical, containing no adjustable parameters, theoretical model is s uggested 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 o f multiquantum transitions between vibrational states, and it requires eval uation of collisional energy transfer to classical oscillator. Three-dimens ional classical energy transfer models in both free-rotation and impulsive (sudden) approximations were used in conjunction with the FHO quantum scali ng. The new theory describes the role of various degrees of freedom in diss ociation both qualitatively and quantitatively. One of the predictions is t hat at low and moderate temperatures, dissociation is strongly preferential , with state-specific rates sharply increasing with vibrational energy; how ever, at high temperatures, the rate dependence on vibrational energy becom es 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 th e literature. (C) 2000 American Institute of Physics. [S0021-9606(00)51741- 3].