## Abstract

The rate of decomposition of carbon disulphide in shock waves through CS$_2$ + Ar mixtures has been studied spectroscopically between 2250 and 3350 $^\circ$K. Mainly, the rate of appearance of CS has been measured, but in some instances it has been possible also to follow the rate of appearance of S$_2$ and the rate of disappearance of CS$_2$. The rate obeys mixed second-order kinetics of the type $-d[CS_2]/dt = k_\infty [\mathrm A r] [\mathrm{CS}_2]+k_0[\mathrm{CS}_2]^2.$ The term that is second-order in [CS$_2$] appears to correspond to a unimolecular decomposition of the CS$_2$ in which the collision partner is another CS$_2$, rather than to a true bimolecular reaction. k$_0$ is found to be about 20 times as large as k$_\infty$. The rate constant for decomposition at infinite dilution in argon is given in Arrhenius form by k$_\infty$ = 10$^{15.9}$ exp (-81.8 kcal/RT) cm$^3$ mole$^{-1}$ s$^{-1}$. The application of R.R.K.H. theory to this yields as the most probable result, $k_\infty = (PZ/3!) (96 kcal/RT)^3 exp (-96 kcal/RT),$ in which P is about 0.1. Thus four effective oscillators may be contributing energy to the decomposition process, the rate-determining step of which is likely to be a transition from the ground state to an excited singlet state at a crossing point located at an energy of about 96 kcal. In this respect CS$_2$ is an interesting contrast to CO$_2$, where the transition is probably singlet-triplet. The firmness of these conclusions is limited by the scarcity of knowledge concerning excited electronic states of CS$_2$.