The photolysis of sulfur dioxide in the presence of foreign gases: IX. allene

Abstract

SO 2 was photoexcited at 25 °C and 313.0 nm in the presence of allene. The quantum yields of the gas phase products, C 2H 4 and CO, were determined over a wide range of allene and SO 2 pressures as well as in the presence of CO 2, NO, and H 2O. The quantum yield of C 2H 4, Φ {C 2H 4}, increases with the ratio [allene]/[SO 2] to a maximum value of 13.1 × 10 −3. In the presence of excess CO 2, NO, or H 2O, or at higher allene pressures, Φ {C 2H 4} is reduced for any [allene]/[SO 2] ratio. At higher [allene]/[SO 2] ratios NO is a much less efficient quencher than at low ratios and thus both singlet and triplet states of SO 2 must participate in C 2H 4 production. The quantum yield of CO, Φ {CO}, also increases with the ratio of [allene]/[SO 2] maximum value of 20.9 × 10 −3. At low values of the ratio [allene]/[SO 2] as well as low total pressure, Φ {CO} is quenched initially by adding CO 2, NO, and H 2O in a manner similar to Φ {C 2H 4}. For continued increases in the pressures of CO 2, NO, and H 2O or at higher values of [allene]/[SO 2], Φ {CO} is enhanced. Both the singlet and triplet emitting states, as well as a previously proposed non-emitting triplet state are necessary to interpret the photochemistry of SO 2 in this study. A relatively complete mechanism is proposed, all of the pertinent rate coefficient ratios are derived and tabulated, and from these values Φ {C 2H 4} and Φ{CO} values are computed which agree well with the observed values.