Flowing Afterglow Studies of the Temperature Dependencies for Dissociative Recombination of O₂⁺, CH₅⁺, C₂H₅⁺, and C₆H₇⁺ with Electrons

Abstract

A temperature-variable flowing afterglow with an electrostatic Langmuir probe has been used to determine the rate coefficients,α e, for the recombination of a series of molecular ions with electrons. The α e for O 2 +, CH 5 +, C 2 H 5 +, and C 6 H 7 + have been determined at temperatures ranging from 80 to 600 K. Data for O 2 + over the temperature range of 100−500 K establish a single-power law dependence, which is consistent with previous data in this temperature range. The hydrocarbon data show that the α e are large at room temperature, being 1.1 × 10 -6, 1.2 × 10 -6, and 2.4 × 10 -6 cm 3 s -1 for CH 5 +, C 2 H 5 +, and C 6 H 7 +, respectively, and exhibit significant dependencies on temperature consistent with theoretical models based on the direct and indirect mechanisms. The change between these two dependencies occurs at a temperature of ∼300 K. The dissociative recombination of these ions is significant to molecular synthesis in interstellar clouds and the ionosphere of Titan, which is to be probed by the NASA Huygens-Cassini spacecraft that reaches the Saturnian system in July 2004. The relevance of the present data to these media is briefly discussed.