Dissociative recombination of organic molecular ions of relevance for interstellar clouds and Titan's upper atmosphere

This thesis presents experimental studies on the dissociative recombination (DR) of the organic molecular ions CD3CND+, CH2CHCNH+, CH3CH2CNH+, CD3CDO+, CH3CHO+ and DCOOD2+. The experiments were all performed at the heavy ion storage ring CRYRING at the Manne Siegbahn Laboratory in Stockholm, Sweden. DR is the process in which a singly charged molecular cation captures a free electron, forming a highly excited intermediate molecule which then dissociates into exclusively neutral fragments. The process plays an important role as a plasma neutralizing mechanism in many cold, low density plasmas such as those encountered in planetary ionospheres and interstellar clouds. DR can also act as the final step in the gas-phase synthesis of different neutral molecules in such environments. Our experimental findings indicate that nitriles that are lost by protonation in Titan’s upper atmosphere or in interstellar clouds to a large extent may be recycled by DR. Also, it appears that the DR of nitrile ions does not break the C-N bond, which supports the hypothesis that nitriles which are formed in Titan’s upper atmosphere do not degrade to recover N2. For the studied acetaldehyde cations, CD3CDO+ and CH3CHO+, we observed a considerable isotopic effect in the cross section, with the lighter isotopologue being more reactive. In the DR of DCOOD2+ an upper limit of only 13% for the branching fraction of the DCOOD + D channel was found. This finding has pronounced effects on the predicted abundance of formic acid in dark clouds