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±_2.1^4.5 × 10^-8 cm³ s^-1 and 2.6 ± 0.8 × 10^-7 cm³ s^-1, respectively, whereas D₂Cl⁺ is roughly half as fast as H₂Cl⁺ with k_DR = 1.1 ± 0.3 × 10^-7 cm³ s^-1 (2σ confidence intervals). DCl⁺ recombines with a rate coefficient below the approximate detection limit of the method (≲5 × 10^-8 cm³ 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.