## Abstract

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.