C3H3+ isomers: temperature dependencies of production in the H3+ reaction with allene and loss by dissociative recombination with electrons.

A technique has been developed to simultaneously determine recombination rate coefficients, alpha e, and initial concentrations of ion types that coexist in a flowing afterglow plasma. This was tested using the H3(+) + allene reaction in which two different C3H3+ isomers are produced. Use of an electrostatic Langmuir probe enabled the C3H3+ isomer branching ratios for propargyl and cyclic C3H3+ from this allene reaction and their alpha e to be determined over the temperature range 172-489 K. The study showed that the cyclic C3H3+ to propargyl C3H3+ branching ratios from the allene reaction varied from 50/50 at 172 K to 18/82 at 489 K. Over this temperature range, the alpha e for both isomers change only slightly. The room temperature alpha e values for propargyl and cyclic C3H3+ are (1.15 +/- 0.2) x 10(-7) and (8.00 +/- 0.1) x 10(-7) cm3/s, respectively. The data are discussed relative to current theories and in relation to fuel-rich flame chemistry, interstellar molecular synthesis, and modeling of Titan's atmosphere.