Nonequilibrium velocity distribution and dissociation rate in dissociation of diatomic molecules

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Abstract
The nonequilibrium velocity distribution and dissociation rate in the dissociation of diatomic molecules in a heat bath of inert atoms are studied by solving the Boltzmann equation with the Monte Carlo simulation. The explicit time-dependent velocity distribution and dissociation rate from the initial equilibrium to the quasisteady state are obtained for the dissociation model, where the vibrational distribution is taken to be the Boltzmann equilibrium distribution; the effects of the vibrational transition are taken into account by a simplified model, and the cross section is taken to be a hard sphere model with the line-of-centers dissociation cross section. The quasisteady state velocity distributions and dissociation rates of upper vibrational levels, from which the dissociation occurs effectively, indicate significant deviations from equilibrium. The decreases in the dissociation rates of the upper vibrational levels from the equilibrium dissociation rates amount to 75% for H2–Ar and 20% for N2 or O2–Ar, which are comparable with the decrease in the dissociation rate due to the nonequilibrium vibrational distribution. For halogens–Ar, the decreases in the dissociation rates are less than 20%. The decrease in the overall dissociation rate from the equilibrium dissociation rate due to the nonequilibrium velocity distribution is 60–30% for H2–Ar at temperatures 1500–4000 °K, where the nonequilibrium dissociation rate is still larger than the experimental rates. The difference between the nonequilibrium dissociation rate and the experimental rates may be attributed to the nonequilibrium vibrational distribution. It is of interest that the nonequilibrium of the dissociation rate due to the nonequilibrium velocity distribution takes place even at low temperatures, where the nonequilibrium of the vibrational distribution may not occur.