Licentiate thesis: Dissociative Recombination of Astrochemically Relevant Organic Molecular Ions

by Erik Vigren (Stockholm University, Department of Physics)

Friday 14 Nov 2008, 14:00 → 16:00 Europe/Stockholm

FA32

Dissociative recombination (DR) is a process in which a molecular cation captures an electron, forming a highly excited intermediate molecule which rapidly dissociates into neutral fragments (atoms or molecules). In low-density plasmas, which are cold enough to contain molecular ions, where the collision energies are small and three-body processes are unimportant, DR is an important neutralizing process. Such plasmas are encountered in, for example, interstellar clouds and planetary ionospheres. The role of DR in these environments is not only to remove molecular cations and free electrons but also to function as a final step in the synthesis of many neutral molecules. This thesis is devoted to experimental investigation into the DR of the acetaldehyde cation (CD3CDO+), protonated acetonitrile (CD3CND+) and protonated acrylonitrile (CH2CHCNH+). These studies were performed at the heavy ion storage ring CRYRING at the Manne Siegbahn Laboratory. The reactions have been shown to have thermal rate coefficients of 9.2 10-7 (T/300)-0.72, 8.1 10-7 (T/300)-0.69 and 1.8 10-6 (T/300)-0.80 cm3 s-1, respectively. While a break of a bond between heavy atoms dominate in the DR of CD3CDO+, the heavy atom structure is more frequently preserved in the DR of CD3CND+ and CH2CHCNH+. Chemical evolution models of the dark cloud TMC-1 have been run using the UMIST code for astrochemistry. Calculated abundances of nitrile molecules are compared with observations. Implications of our findings on the chemistry in Titans atmosphere are discussed as well.