Collisional removal of atomic carbon, C[2p(2)(P-3(J))], by aldehydes and ketones, investigated by time-resolved atomic resonance absorption spectroscopy in the vacuum ultra-violet

A kinetic study is presented of the collisional removal of ground state ato mic carbon, C[2p(2)(P-3(J))], With various aldehydes and ketones in the gas phase following pulsed irradiation. The atomic carbon was generated by the photolysis of C3O2 (lambda > Ca. 160 nm) in the presence of excess helium buffer gas and the added reactant gases in a slow flow system, kinetically equivalent to a static system, and monitored photoelectrically by time-reso lved atomic resonance absorption spectroscopy in the vacuum ultra-violet at lambda = 166 nm (3(3)P(J) <-- 2(3)P(J)) using signal averaging techniques. Absolute second-order rate constants (k(R)/cm(3) molecule(-1) s(-1), 300 K ) for the removal of C(23PJ) with these reactants were found to be as follo ws: formaldehyde 6.2 +/- 0.3 x 10(-10); acetaldehyde 5.4 +/- 0.3 x 10(-10); propionaldehyde 4.1 +/- 0.3 x 10(-10); n-butyraldehyde 6.6 +/- 0.3 x 10(-1 0); pentanal 4.6 +/- 0.2 x 10(-10); hexanal 5.3 +/- 0.4 x 10(-10); acetone 5.9 +/- 0.3 x 10(-10); butanone 5.1 +/- 0.2 x 10(-10); 2-pentanone 3.8 +/- 0.2 x 10(-10); and 3-pentanone 4.6 +/- 0.1 x 10(-10.) No significant monoto nic variation is thus observed in the rate data within these series of coll isional processes where, from the similarity in the observed results, it is concluded that reaction is dominated by attack on the carbonyl group. The large values of these rate constants indicate that reactions of C(23PJ) Wit h aldehydes and ketones, some of which have been observed by radio frequenc y spectroscopy in interstellar clouds and considered to be generated initia lly by hot atom reactions, are sufficiently rapid to be included in modelli ng of the interstellar medium. (C) 1999 Elsevier Science S.A. All rights re served.