Dissociative recombination of HCl, HCl, DCl, and DCl in a flowing afterglow.

Dissociative recombination of electrons with HCl, HCl, DCl, and DCl 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 HCl and DCl employed both VENDAMS and the more traditional technique of monitoring electron density as a function of reaction time. At 300 K, HCl and HCl recombine with k = 7.7± × 10 cm s and 2.6 ± 0.8 × 10 cm s, respectively, whereas DCl is roughly half as fast as HCl with k = 1.1 ± 0.3 × 10 cm s (2σ confidence intervals). DCl recombines with a rate coefficient below the approximate detection limit of the method (≲5 × 10 cm s) at all temperatures. Relatively slow dissociative recombination rates have been speculated to be responsible for the large HCl and HCl abundances in interstellar clouds compared to current astrochemical models, but our results imply that the discrepancy must originate elsewhere.