Dissociative recombination of HCl+, H2Cl+, DCl+, and D2Cl+ in a flowing afterglow

Dissociative recombination of electrons with HCl+, H2Cl+, DCl+, and D2Cl+ has been measured under thermal conditions at 300, 400, and 500 K using a flowing afterglow-Langmuir probe apparatus. Measurements for HCl+ and DCl+ employed the variable electron and neutral density attachment mass spectrometry (VENDAMS) method, while those for H2Cl+ and D2Cl+ employed both VENDAMS and the more traditional technique of monitoring electron density as a function of reaction time. At 300 K, HCl+ and H2Cl+ recombine with k(DR) = 7.7 +/-(4.5)(2.1) x 10(-8) cm(3) s(-1) and 2.6 +/- 0.8 x 10(-7) cm(3) s(-1), respectively, whereas D2Cl+ is roughly half as fast as H2Cl+ with k(DR) = 1.1 +/- 0.3 x 10(-7) cm(3) s(-1) (2 sigma confidence intervals). DCl+ recombines with a rate coefficient below the approximate detection limit of the method (<5 x 10(-8) cm(3) s(-1)) at all temperatures. Relatively slow dissociative recombination rates have been speculated to be responsible for the large HCl+ and H2Cl+ abundances in interstellar clouds compared to current astrochemical models, but our results imply that the discrepancy must originate elsewhere.