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

摘要

A kinetic study is presented of the collisional removal of ground state atomic carbon, C[2p~2(~3p_J)], with various aldehydes and ketones in the gas phase following pulsed irradiation. The atomic carbon was generated by the photolysis of C_3O_2 (#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-resolved atomic resonance absorption spectroscopy in the vacuum ultra-violet at #lambda#=166 nm (3~3P_J<-2~3P_J) using signal averaging techniques. Absolute second-order rate constants (k_R/cm~3 molecule~(-1)s~(-1), 300K) for the removal of C(2~3P_J) with these reactants were found to be as follows: formaldehyde 6.2+-0.3 x 10~(-10); acetaldehyde 5.4+-0.3x10~(-10); propionaldehyde 4.1+-0.3 x 10~(-10); n-butyraldehyde 6.6+-0.3x10~(-10); 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 monotonic variation is thus observed in the rate data within these series of collisional 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(2~3P_J) with aldehydes and ketones, some of which have been observed by radio frequency spectroscopy in interstellar clouds and considered to be generated initially by hot atom reactions, are sufficiently rapid to be included in modelling of the interstellar medium.