NEGATIVE TEMPERATURE COEFFICIENT OF ATOMIC RECOMBINATION RATE CONSTANTS FROM FLASH PHOTOLYSIS AND SHOCK-WAVE DATA . BROMINE

The bromine atom recombination rate constants, krf1s, obtained in flash photolysis experiments between 300 and 1300°K (preceding paper) were compared with the dissociation rate constants of Br2, kdsh's, obtained in shock-wave experiments between 1250 and 2300°K. It was found that the phenomenological equation k d,sh/kr,f1 = K, where K is the equilibrium constant, is not obeyed if the kdsh is measured by the method of absorption spectroscopy. On the other hand, if kd,sh is measured by the emission spectroscopy method [R. K. Boyd, G. Burns, T. R. Lawrence, and J. H. Lippiatt, J. Chem. Phys. 49, 3804 (1968)] the phenomenological equation is obeyed at 1300°K. The negative temperature coefficients from shock-wave experiments are considerably higher than those from flash photolysis at comparable temperatures. These conclusions are drawn by comparison of more than 200 measurements of kr,f1 (preceding paper) with more than 200 measurements of kr,f1 obtained by previous investigators. All these experimental findings are shown to be consistent with a simple model for dissociation of Br2 in shock waves. The model is based on the assumption that the Bethe-Teller law, which holds for a variety of vibrationally relaxing systems, is valid in the early stages of Br2 dissociation, which occurs from the uppermost vibrational levels. The model predicts that true phenomenological rate coefficients for the Br2 dissociation reaction, kr,f 1, differ from the equilibrium rate constants, kr,f 1, and that the ratio of kr,f1 taken at the same translational temperature, decreases rapidly with T. If argon is used as a third body, this ratio is 0.96 at 1300°K, while at 2300°K the nonequilibrium rate coefficient is only 33% of kr,f1.