Reactions of atomic carbon, C[2 (3P J)],with dienes and diynes investigated by time-resolved atomic resonance absorption spectroscopy in the vacuum ultraviolet

Abstract

Absolute data for the collisional removal of ground-state atomic carbon, C[2 (³P_J)], generated by pulsed irradiation and monitored by time-resolved atomic resonance absorption spectroscopy in the vacuum ultraviolet, have been determined for reaction with a range of dienes and diynes. The precursor of atomic carbon was C₃O₂ (λ>ca. 160 nm), photolysed in the presence of excess helium buffer gas and the added reactant gases in a slow flow system, kinetically equivalent to a static system. C(2³P_J) was then monitored photoelectrically at λ=166 nm (3³P_J←2³P_J) with data capture and signal averaging of complete atomic profiles by a transient digitiser with direct computer interfacing for kinetic analysis. The following absolute second-order rate constants (k_R/c molecule^-1 s^-1, 300 K) for the reactions of ground-state carbon atoms with the various organic targets have been measured: allene, 8.0±0.1×10^-10; buta-1,3-diene, 1.1±0.1×10^-9; penta-1,4-diene, 1.2±0.1×10^-9; hexa-1,5-diene, 1.2±0.1×10^-9; hepta-1,6-diene, 1.2±0.1×10^-9; octa-1,7-diene, 1.2±0.1×10^-9; nona-1,8-diene, 1.3±0.1×10^-9; deca-1,9-diene, 1.4±0.1×10^-9; octa-1,7-diyne, 1.1±0.1×10^-9; nona-1,8-diyne, 1.1±0.1×10^-9. These absolute rate data are compared with data reported hitherto for the reaction of C(2³P_J) with the analogous mono-alkenes and mono-alkynes. The large value of the rate constants for reactions of C(2³P_J) with both the mono- and di-organic species indicate cross-sections which are sufficiently large to be included in the modelling of interstellar clouds characterised by low temperatures and densities, an aspect of this work which is considered.