Studies of methylene chemistry by pulsed laser-induced decomposition of ketene. Part 1.—Ketene in the presence of noble gases

Abstract

Gaseous mixtures of ketene with argon (or xenon) have been photodecomposed at room temperature by 308 nm u.v. radiation from a pulsed exciplex laser. At low conversions the major products are ethylene, acetylene, hydrogen and carbon monoxide. Minor products include methane, ethane, propene and allene. Kinetic modelling based on the Gear algorithm has been used to predict the product distribution and its pressure dependence. The modelling reveals that: (a) the major product of the recombination process 2³CH₂→ C₂H₂+ H₂(2H) is molecular and not atomic hydrogen; (b) the disproportionation process 2³CH₂→ CH₃+ CH is probably occurring; (c) primary quantum yields are as follows: CH₂CO +hν(308 nm)→ CH₂(¹A₁)+ CO ϕ= 0.6 ± 0.1, → CH₂(³B₁)+ CO ϕ= 0.4 ∓ 0.1; (d) for the reaction ¹CH₂+ CH₂CO → C₂H₄+ CO k= 2.1^+0.7_–0.5× 10^–10 cm³ molecule^–1 s^–1; (e) propene and allene are formed via the intermediacy of the vinyl radical, and secondary reactions such as ³CH₂+ C₂H₄→ C₃H₆, +C₂H₂→ C₃H₄ are unimportant (rate constants are < 10^–15 cm³ molecule^–1 s^–1).

These findings are compared and contrasted with previous flash-photolysis and other studies of ketene photodecomposition.