Kinetic spectroscopy in the vacuum ultra-violet region I. The dissociation energy of SO and the combustion of hydrogen sulphide, carbon disulphide and carbonyl sulphide

Abstract

A flash-photolysis apparatus is described which is capable of detecting transient reaction intermediates by means of their absorption spectra in the vacuum ultra-violet region. With this equipment studies have been made of the combustion of carbon disulphide, hydrogen sulphide and carbonyl sulphide under adiabatic and isothermal conditions. At the shortest delay time obtainable, a new spectrum appears in all of the reactions studied, in the wavelength range 2400 to 1900 Å. This spectrum has been shown to be that of the SO radical and in the case of the hydrogen sulphide reactions the spectrum can be seen to merge into a dissociation continuum, the convergence limit lying at 53677 cm -1 , corresponding to a dissociation energy for SO of 127.1 kcal/mole. Irregularities in the vibrational intervals of the 3 Σ- upper state of SO show that some of the levels are strongly perturbed, indicating a crossing or attempted crossing of this 3 Σ- curve with another potential curve. This is confirmed in the plot of Δ G v'+1/2 against v ' which has a point of inflexion between v ' = 14 and v ' = 17. It is shown that the potential curve responsible for these vibrational perturbations is the 3 II which is also responsible for the predissociation previously observed by Martin in the emission spectrum of SO, and that this state must dissociate to ground-state atomic products, i.e. O( 3 P ) and S( 3 P ). A new transient spectrum has been observed in the wavelength range 1840 to 1740 Å in the hydrogen sulphide and carbon disulphide reactions and these same bands have also been detected in the flash photolysis of SO 2 . It has been possible to fit the bands into a vibrational scheme and to assign them to an isomeric form of SO 2 . Reaction mechanisms for the combustion of H 2 S, CS 2 and COS and a mechanism for the formation of SO 2 are discussed.