Negative Temperature Coefficient of Atomic Recombination Rate Constants from Flash Photolysis and Shock-Wave Data. Bromine

The bromine atom recombination rate constants, $k_{\mathrm{r,fl}}\text{'}\mathrm{s}$ , obtained in flash photolysis experiments between 300 and 1300°K (preceding paper) were compared with the dissociation rate constants of Br₂, $k_{\mathrm{d,sh}}\text{'}\mathrm{s}$ , obtained in shock‐wave experiments between 1250 and 2300°K. It was found that the phenomenological equation $k_{\mathrm{d,sh}}{\mathrm{\hspace{0.17em}/\hspace{0.17em}k}}_{\mathrm{r,fl}}\mathrm{\hspace{0.17em}=\hspace{0.17em}K}$ , where $K$ is the equilibrium constant, is not obeyed if the $k_{\mathrm{d,sh}}$ is measured by the method of absorption spectroscopy. On the other hand, if $k_{\mathrm{d,sh}}$ 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 $k_{\mathrm{r,fl}}$ (preceding paper) with more than 200 measurements of $k_{\mathrm{d,sh}}$ 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, $k_d\text{'}\mathrm{s}$ , differ from the equilibrium rate constants, $k_{\mathrm{d,eq}}\text{'}\mathrm{s}$ , and that the ratio of $k_d{\mathrm{\hspace{0.17em}/\hspace{0.17em}k}}_{\mathrm{d,eq}}$ , 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 $k_{\mathrm{d,eq}}$ .