The kinetics of elementary reactions involving the oxides of sulphur II. Chemical reactions in the sulphur dioxide afterglow

The main recombination reactions in the sulphur dioxide afterglow are shown to be \begin{equation*}\tag{1} \mathrm{O + SO}_2 + M = \mathrm{SO}_3 + M\end{equation*} and \begin{equation*}\tag{2} \mathrm{O + SO} + M = \mathrm{SO}_2 + M,\end{equation*} with rate constants of (4$\cdot$7 $\pm$ 0$\cdot$8) x 10$^{15}$ and (3$\cdot$2 $\pm$ 0$\cdot$4) x 10$^{17}$ cm$^6$ mole$^{-2}$ s$^{-1}$ respectively at 300 $^\circ$K for M = Ar. Reaction (2) is the dominant process removing sulphur monoxide (SO) which is otherwise remarkably unreactive. The absolute intensity of the sulphur dioxide afterglow is found to be $$I = 1\cdot5 x 10^8[\mathrm{O}][\mathrm{SO}] \mathrm{cm}^3 \text{mole}^{-1} \mathrm{s}^{-1}$$ for argon carriers at pressures between 0$\cdot$25 and 3$\cdot$0 mmHg. The afterglow emission comes from three excited states of SO$_2$. Spectroscopic and kinetic studies show that these are populated subsequent to or by the third order combination reaction (2). Excited SO$_2$ is removed mainly by electronic quenching.