## Abstract

The rate of increase in strength of absorption bands of SO has been measured in shock-heated mixtures of sulphur dioxide and argon. Arrhenius-type plots indicate a unimolecular first step of the order $d[\mathrm{SO}]/dt = k[\mathrm{SO}_2][\mathrm{M}],$ where [SO], [SO$_2$] and [M] are concentrations of [SO], [SO$_2$] and total gas. The apparent activation energy at around 3500 $^\circ$K is 56 kcal/mole. It is shown that on unimolecular reaction theory, if four harmonic modes of oscillation in the SO$_2$ molecules contribute to the energy available for transformation, the true activation energy is 74 kcal/mole. This agrees with the energy of excitation to a known triplet state of SO$_2$, and on this basis it is suggested that the first steps in the decomposition are \begin{equation*}\tag{1}\mathrm{SO}_2+\mathrm{M} = \mathrm{SO}^*_2 + \mathrm{M} - 73\cdot6 kcal/\text{mole}\end{equation*} and \begin{equation*}\tag{2}\mathrm{SO}^*_2 + \mathrm{SO}_2 = \mathrm{SO}_3 + \mathrm{SO} + 25\cdot6 kcal/\text{mole}\end{equation*}. Step (2) is spin-allowed, whereas the more direct reaction SO$_2$ + SO$_2$ = SO$_3$ + SO - 48 kcal/mole is spin-forbidden. This is an unusual type of decomposition mechanism and occurs because of the high dissociation energy of SO$_2$, because the direct step of low-energy is spinforbidden, and because there is a favourably situated triplet state of the molecule.