Trajectory Studies of Atomic Recombination Reactions. II. The Energy Transfer Mechanism and Nonequilibrium Effects

Abstract

Rate constants for the recombination of bromine atoms in excess of the inert gases helium, argon and xenon have been computed by 3D classical trajectory/Monte Carlo methods. Previous work on the radical molecule complex (RMC) mechanism [J. Chem. Phys. 55, 4717 (1971)] has been extended by the calculation of the recombination rates via the energy transfer (ET) mechanism Br+Br⇌ Br2*, Br2*+M→ Br2+M. Nonequilibrium corrections for the two mechanisms were estimated by computing the fraction of initially formed, highly excited, Br2 molecules which redissociated on subsequent collisions with third body atoms. Rate constants were determined for each mechanism and each third body at 300, 500, 1000, and 1500°K and for argon at 2000°K. The interaction potential between Br and M was assumed to be of the Lennard-Jones form with the following parameters. M = He, ε = 0.5 kcal/mole, σ = 2.5 A; M = Ar, ε = 1.0 kcal/mole, σ = 3.0 Å; M = Xe, ε = 1.0 kcal/mole, σ = 3.5 Å. The RMC mechanism gives rate constants with appreciable negative temperature coefficients, while the ET rate constants vary slowly with temperature. The RMC mechanism is dominant for argon and xenon at low temperatures and the ET mechanism is dominant for argon and xenon at high temperatures and for helium over the whole temperature range studies. The nonequilibrium corrections reduced the rate constants by more than one-half in some cases. The total recombination rates are in good agreement with experiment at low temperatures but above 1000°K they exceed the experimental rates obtained using flash photolysis by a factor not larger than two.