Flowing afterglow studies of temperature dependencies for electron-ion dissociative recombination of hydrocarbon ions
McLain, Jason Lee
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A temperature-variable flowing afterglow with an electrostatic Langmuir probe (VT-FALP) has been used to determine the rate coefficients, ±e, for the dissociative recombination ++++(DR) of a series of molecular ions with electrons. The ±e for O2, CH5, C2H5, c-C3H3, l-+++++++C3H3, c-C6H7, HCNH, CH3CNH, CH3CH2CNH, (HCN)2H, (CH3CN)2H, and +(CH3CH2CN)2H have been determined at temperatures ranging from 80 to 600 K. The DR of these ions is important to molecular synthesis and the ionization balance in interstellar clouds and the ionosphere of Titan, which has been probed by the Cassini-Huygens spacecraft since +2004. The DR of O2 was the standard calibration test for recombination measurements. The + ±e’s for O2over the temperature range of 100-500 K followed a single power law dependence of -0.7T, which is consistent with previous data in this temperature range. A series of different types of molecular ions were chosen to study for which the ±e and their temperature dependence have not been obtained previously, and which were relevant to the applications. These temperature -0.5 to -1.5dependencies are represented as power laws in the range of T. Until recently, most recombination measurements have been obtained only at room temperature. However, typically the environments where DR is important are not at this temperature. The ±e’s obtained in the present studies exhibit significant dependencies on temperature usually consistent with simple theoretical models based on a combination of the direct and indirect mechanisms. The production of proton bound cluster ions has also been investigated. These cluster ions are produced by association when large reactant concentrations are used. Here, this association can compete with DR of the protonated ions, creating a plasma containing two simultaneously recombining ions. All of the ±e’s vary significantly depending on the ion type. The number of atoms, type of bonding, and whether the ions are cyclic or linear can all affect the ±e. The relevance of the present data to the applications is also discussed.