Excitation of the $B^2\Sigma_{1/2}^+$ state of the HgBr* molecules in a gas-discharge plasma formed from a mixture of mercury dibromide and helium

An experimental investigation was made of the formation of negative Br$^-$ ions in an HgBr$_2$–He mixture used as the active medium in the earlier studies of the HgBr laser. The rate constant of the formation of Br$^-$ was (14 – 7) $\times$ 10$^{-11}$ cm$^3$ s$^{-1}$ in the range of $E/N$ from $2\times 10^{-17}$ to $4 \times 10^{-16}$ V cm$^2$. A comparison of these results with our earlier theoretical data revealed a relationship between the products Br$^-$ and Br, e of the processes of excitation of the $B^2\Sigma_{1/2}^+$ state of the HgBr* molecules in a gas-discharge plasma formed from an HgBr$_2$–He mixture (with the degree of ionisation not exceeding 10$^{-6}$). The electron excitation of the HgBr* molecules, resulting in the formation of Br$^-$, occurred mainly at low values of $E/N$ (below 3 $\times$ 10$^{-17}$ V cm$^2$); an increase in $E/N$ reduced the fraction of Br$^-$ (compared with Br, e) and for $E/N = 4\times 10^{-16}$ V cm$^2$, this fraction was $\sim$1%.