Laser flash photolysis of 1-alkenes at 193 nm in the gas phase: effect of molecular size on the formation yield of allyl radical

Abstract

Allyl radical has been detected as a major product in the photolysis of 1-alkenes (CH 2CHCH 2R)using an ArF excimer laser (193 nm). A broad absorption spectrum was obtained immediately after excitation, indicating the formation of a vibrationally hot radical. This spectrum is gradually sharpened by collisional deactivation. The relaxed spectrum in the presence of high pressure nitrogen is almost the same as that reported by flash photolysis on the microsecond time scale. The relative yields of allyl radical have been estimated by measuring absorbances at the peak (223 nm). In the presence of 800 Torr nitrogen, they decrease as the size of the molecule becomes larger in the order of C 3H 6>1-C 4H 8>1-C 5H 10⪢1-C 6H 12H, 4-CH- 31-C 5H 9, 1-C 7H 14⪢1-C 8H 16>1-C 9H 18. This size dependence is discussed in terms of a “hot molecule mechanism”, namely it is postulated that 1-alkenes are dissociated from hot molecules formed by internal conversion.