Nonequilibrium Dissociation Rates Research Articles

Vibrational nonequilibrium dissociation of Br2 in collisions with Ar and Br atoms

The collision induced dissociation processes in dilute mixtures of Br2–Ar and Br2–Br are investigated. Vibrational transition rates and dissociation rates from individual vibrational levels are derived from quasiclassical Monte Carlo trajectory calculations where only vibrational nonequilibrium effects are considered. The full transition rate matrix is completed by a moment analysis of the trajectory results. From the steady state solution of vibrational-level master equations, it is found that steady state populations in excited vibrational states are largely depleted but the nonequilibrium dissociation rates are about 30%–75% less than the equilibrium rates. The low activation energy observed in the previous experiments in the dissociation process, Br2+Br→Br+Br+Br cannot be explained by the present analysis.

Nonequilibrium vibrational populations and dissociation rates of HCl under IR laser pumping

Nonequilibrium vibrational distributions and dissociation rates of HCl under IR laser pumping of suitable frequency have been calculated by solving a system of vibrational master equations including optical pumping, V-V (vibration-vibration) and V-T (vibration-transition) energy exchanges. The results show that under a laser pump of 106s−1 dissociation of HCl can occur as a result of the V-V up-pumping mechanism, which transports up to the top of the HCl vibrational ladder the vibrational quanta introduced by the laser pump over the first few vibrational levels of HCl. The influence of the gas temperatureT on the dissociation rate is discussed either for the isothermal problem or for the nonisothermal one. At high pumping rates an increase ofT decreases the dissociation rate (300 K<T<1000 K) as a result of the destruction of V-V up-pumping mechanism at high temperature.

V—V pumping up in non-equilibrium nitrogen: Effects on the dissoviation rate

Non-equilibrium dissociation rates of N 2 in electrical discharges have been calculated by solving a system of vibrational master equations, including e—V (electron—vibration), V—V (vibration—vibration), V—T (vibration—translation) energy exchanges. V—V and V—T rates have been obtained by using a semiclassical model, while the e—V rates have been calculated by averaging the corresponding cross sections with a self-consistent electron energy distribution function. The results show that in the reduced electric field range 3 × 10 −16 V cm 2 ≤ E/N ≤ 6 × 10 −16 V cm 2, n e = 10 11 cm 3, p = 5 Torr, T = 500 K the contribution to the dissociation rate coming from the vibrational mechanism overcomes the corresponding one from electronic dissociative collisions. A satisfactory agreement between the present theoretical results and the experimental ones is found specially at low E/N values.

Nonequilibrium vibrational populations and dissociation rates of oxygen in electrical discharges

Nonequilibrium vibrational distributions and dissociation rates of molecular oxygen in both electrical and thermal conditions have been calculated by solving a system of master equations including V-V (vibration-vibration), V-T (vibration-translation) and e-V (electron-vibration) energy exchanges. The dissociation constant under thermal conditions (i.e. without electrons) follows an Arrhenius law with an activation energy of 120 kcal/mole, while the corresponding rates under electrical conditions (5000 ⩽ T e ⩽ 15000 K, 300 ⩽ T g ⩽ 1000 K, 10 11 ⩽ n e ⩽ 10 12 cm −3,5 ⩽ p ⩽ 20 torr) increase with decreasing gas ( T g) and electron ( T e) temperatures and pressure ( p) and with increasing electron density ( n e). These results are explained on the basis of the different interplay of V-V and V-T energy exchanges and are rationalized by means of simplified models proposed in the literature. The accuracy of the present results is discussed paying particular attention to the dependence of V-V and V-T rate coefficients on the vibrational quantum number. A comparison of the calculated dissociation rates with the corresponding ones obtained by the direct electron impact mechanism shows that the present mechanism prevails at low electron and gas temperatures. Finally a comparison is shown between theoretical and experimental dissociation rates under electrical and thermal conditions.

Nonequilibrium velocity distribution and dissociation rate in dissociation of diatomic molecules

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.