Pressure effects and quantum yields in the photolysis of ethylene and propene at 185 nm

Gaseous ethylene and propene were photolysed with varying amounts of added nitrogen, oxygen, and other gases. The product analysis and variation of yields with added gas are consistent with the major primary reactions (i)–(v) for which the primary quantum yields were estimated. Several other minor primary reactions were found. The C2H4+hν(λ= 185 nm)→ C2H2+ H2; ϕ= 0·68 (i), → C2H2+ 2H; ϕ= 0·20 (ii), → C2H3+ H; ϕ= 0·16 (iii), C3H6+hν(λ= 185 nm)→ CH2:CHCH2·+ H; ϕ= 0·41 (iv), → C2H3·+ CH3; ϕ= 0·36 (v) reactions are inhibited by the addition of inert gases ranging from He to CO2; the inhibition is, within experimental error, the same for each additive. Higher pressures were needed to inhibit the ethylene photolysis than other olefins studied and the effect is attributed to the smaller collision diameter of the excited state in ethylene. The results for propene can be explained by a mechanism involving two excited intermediates, only one of which is readily quenched.