Recombination of bromine atoms between 300 and 6000°K, theory and experiment

Abstract

The dissociation of Br 2 in Ar was studied in the same shock tube using three different time dependent observables which were needed to measure the rate of the reaction: (1) Br 2 molecular absorption, (2) Br atom two-body emission and (3) density gradient change, detected by laser schlieren technique. It was found that the first observable was most useful in determination of dissociation rate constants between about 1500 and 1800°K and the second observable between about 1200 and 1700°K. Use of these two observables yielded rate constants which were in agreement with those earlier experimental data, which are likely to be most reliable. The laser schlieren technique yielded new dissociation rate constants between 2100 and 3000°K. These data were found to be consistent with the “reliable” “emission” and “absorption” data, as well as with the earlier flash photolysis data. In order to interpret the experimental results and to study the dynamics of the reaction, the dissociation of Br 2 in Ar was studied by 3-D classical trajectory calculations at 1500, 2500, 3500, and 6000°K. In agreement with earlier trajectory studies, it was found that Br 2 molecules react only if their total energy is within a few kT of dissociation limit and that metastable molecules, with total energy above the dissociation limit, are particularly reactive. The average energy, E >, and the average angular momentum, l >, transferred in dissociative and non-dissociative collisions were calculated as a function of total energy of Br 2 molecule. Generally, | E >| (non-dissoc.) and | l >| (non-dissoc.) were found to be considerably smaller than the same quantities for dissociative collisions. For energetic metastable molecules, dissociation could be accomplished by a decrease of internal energy and angular momentum of the molecule through collision. Thus, collisionally induced rotational de-excitation provides an additional mechanism for dissociation of diatomic molecules. The non-equilibrium effects in dissociation have been evaluated at 3500 and 6000°K by the method of multiple collisions. The calculated steady state dissociation rate constants between 1500 and 6000°K are in good agreement with the available experimental data. Finally, the validity of the rate quotient law, i.e. k forward / k reverse = K eg , was demonstrated by trajectory calculation technique.