Studies of methylene chemistry by pulsed laser-induced decomposition of ketene. Part 3.—Recent mechanistic developments

Abstract

Kinetic modelling has been used to predict the product distribution in the previously studied 308 nm laser-induced photodecomposition of ketene in the presence of added argon, added ethylene or added acetylene. The mechanism and its associated rate constants have been modified to take account of several recent independent studies. The modelling reveals the following. (a) The reaction ³CH₂+ H → CH + H₂ is the major source of molecular hydrogen and indeed can account for all the molecular hydrogen observed. The reaction is the sole source of CH. Confirmation of the importance of this reaction is obtained from the variation of allene yields with added ethylene. (b) Limits on the primary quantum yields are as follows: [graphic omitted], (c) If ϕ₁= 1 and ϕ₂= 0 then ³CH₂ must be formed from ¹CH₂ by collisional processes with reactive substrates and from the total removal rates of ¹CH₂ with each substrate molecule the following fractions (f) for collisional induced intersystem crossing are: CH₂CO, f= 0.23; C₂H₄, f⩽ 0.23, probable lower limit 0.12; C₂H₂, f⩽ 0.23 probable lower limit 0.16. (d) With added ethylene, C₃H₅ is formed via the decomposition of vibrationally excited C₃H₆ with a rate constant, k= 1.0×10⁷ s^–1, consistent with an RRKM estimate. These findings are in reasonable agreement with other studies which give added support to the improved mechanistic model.