title: H-atom yields from the photolysis of acetylene and from the reaction of C2H with H2, C2H2, and C2H4.
abstract: The photolysis of acetylene at 193 nm has been investigated as a source of the ethynyl radical, C(2)H, for product branching ratio studies, particularly the formation of H atom product as the photolysis, producing a 1:1 ratio of C(2)H and H, provides an internal calibration. Previous literature had suggested that C(2)H and H may only be a minor component of acetylene photolysis at 193 nm. Acetylene was photolyzed at low laser energy densities (<7 mJ cm(-2)), with H atoms being observed as a function of time by VUV laser induced fluorescence. When C(2)H was reacted with C(2)H(2), a reaction that is known to produce H atoms with unit yield, the ratio of photolytic H atom production to chemical production was 0.96 +/- 0.03. The rate coefficient for the reaction of C(2)H with C(2)H(2) could accurately be retrieved from the time evolution of the H atom signal. The results suggest that acetylene photolysis at low laser energies is a good source of C(2)H for product branching studies, and the technique has been applied to the reactions of C(2)H with ethene and propene. For the reaction with ethene between 23 and 81 Torr, the yield of H is 0.94 +/- 0.06, suggesting that an addition elimination mechanism dominates with the formation of vinylacetylene and H atoms. For the reaction of C(2)H with propene, no H atom product was observed, putting a lower limit of <5% for H atom production. Possible explanations for the low H atom yield are discussed. The implications of these results in combustion and planetary atmospheres are briefly considered.
authors.affiliation: School of Chemistry, University of Leeds, Leeds, LS2 9JT, United Kingdom.
authors.forename: Tamás
authors.initials: T
authors.lastname: Kovács
authors.forename: Mark A
authors.initials: MA
authors.lastname: Blitz
authors.forename: Paul W
authors.initials: PW
authors.lastname: Seakins