Dissociation of HBr and Br2 in Shock Waves

The rate of dissociation of HBr has been measured at temperatures in the range 1500–2700°K in shock waves by recording the light emitted when two Br atoms combine to form Br₂(¹II_u). The experiments show that the activation energy for HBr dissociation cannot be substantially different from the HBr bond energy. In particular, the experimental data are consistent with the assumption that the rate coefficient of the reaction $\mathrm{HBr+M}\to \mathrm{H+Br+M},\mathrm{M=Ar}$ or HBr, is ${\text{1.2× 10}}^{19}{\mathrm{\hspace{0.17em}T}}^{\text{−2}}\hspace{0.17em}\exp {\mathrm{(-D}}_{\mathrm{HBr}}{\mathrm{/RT)M}}^{\text{−1}}·{\sec }^{\text{−1}}$ , where D_HBr is the bond dissociation energy of HBr (88 kcal/mole). The rate coefficients for the dissociation of Br₂ by Ar, He, and Br were determined by shock heating mixtures of Br₂ and Ar or He. The results are in agreement with those reported by other workers. The temperature and concentration dependence of the light emitted behind bromine—argon shocks have been examined, and the results are consistent with the premise that over a 1500–3000°K temperature range, the emission at λ595 nm is due to the reaction $2\mathrm{Br}{(}^2{P}_{\text{3/2}}\mathrm{)\rightleftharpoons }{\mathrm{Br}}_2{(}^1{\Pi }_u\mathrm{)\to }{\mathrm{Br}}_2{(}^1{\Sigma }_g^{+}\mathrm{)+h\nu }$ .