Measurements of dissociative recombination coefficients of H+3, HCO+, N2H+, and CH+5 at 95 and 300 K using the FALP apparatus

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Abstract
Meaurements are presented of the dissociative recombination coefficients αt for reactions of electrons with H+3 , D+3 , HCO+, DCO+, N2H+, N2D+, and CH+5 ions at 95 and 300 K. The measurements were made under truly thermalized conditions using a flowing afterglow (FALP) apparatus. Contrary to previous stationary afterglow (SA) studies, αt(H+3) was found to be immeasurably small [≲2 (−8) cm3 s−1] at both temperatures which is consistent with recent theoretical predictions. However some evidence was obtained indicating that vibrationally excited H+3 recombined efficiently, which is also in accordance with the recent theory. At 300 K, αt(HCO+)=1.1 (−7) cm3 s−1 and αt(N2H+)=1.7 (−7) cm3 s−1 and both were larger by a factor of three at 95 K. αt(CH+5) =1.1 (−6) cm3 s−1 at 300 K and 1.5 (−6) cm3 s−1 at 95 K. The αt(HCO+) data are compared with previous SA data and the αt(N2H+) and αt(CH+5) data are discussed in relation to the recombination coefficients αe(N2H+) and αe(CH+5) derived from merged beam (MB) cross section data. Stressed throughout the paper is the need to appreciate the chemical and physical processes which can occur in the afterglow and which may distort αt determinations. Tentative explanations are given for the differing values of αt and αe obtained from FALP, SA, and MB experiments.