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

Abstract: 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.