Trajectory Studies of Atomic Recombination Reactions

Abstract

The radical molecule complex (RMC) theory for bromine atom recombination in the presence of an inert gas M has been tested by computing 3D classical trajectories for $\mathrm{BrM}+\mathrm{Br}$ collisions. The dissociation energy of BrM was taken to be of the order of one kcal/mole. Monte Carlo methods were used to select random initial conditions of the BrM molecules with bound and metastable states included. The largest cross sections for the recombination reaction were with deep potential wells and small collision diameters for the $\mathrm{Br}–\mathrm{M}$ interaction. Heavy third bodies were slightly more effective than light third bodies in promoting the reaction. The velocity averaged cross sections decreased with temperature as T^−0.4 for M=Xe, T^−0.6 for M=Ar and T^−0.9 for M=He. Recombination rate constants, k_r, were calculated at 300, 600, 1000, and 1500°K. For argon, with a $\mathrm{Br}–\mathrm{Ar}$ well depth of 1.0 kcal/mole, the absolute magnitude and temperature dependence of k_r agreed with experiment. For helium and xenon agreement of the calculated and experimental k_r values at 300°K was obtained with the same well depth, 1.0 kcal/mole. The temperature dependence of k_r was reasonable for Xe but for He the calculated value of k_r at 1000°K was more than a factor of two smaller than the experimental value. The limitations of the RMC model are discussed in the light of these findings. The energy distributions for the bromine molecules formed in the recombination reaction show that the mean total internal energy is close to the dissociation energy but that there are wide spreads of rotational and vibrational energies; recombination does not take place predominantly into a few vibrational levels near the dissociation limit. The trajectory results are compared with the findings of Blake, Browne, and Burns [J. Chem. Phys. 53, 3320 (1970)] who used a Sutherland potential model for the $\mathrm{BrM}–\mathrm{Br}$ interaction.