In this thesis, the electron impact dissociation of molecular ions has been studied using the ion storage ring CRYRING at Manne Siegbahn Laboratory in Stockholm, Sweden. The electron impact dissociation of molecular ions occurs via many processes, which lead to different charge states of the product fragments. These processes are dissociative recombination (DR), dissociative excitation (DE), resonant ion-pair formation (RIP) and dissociative ionization (DI). In this thesis the first three of these processes have been studied for molecular ions of astronomical and atmospherical interest.

New experimental procedures, measuring techniques and instrumentation have been developed in order to improve the reliability of the achieved results and also to be able to study the different processes mentioned above. The HeH⁺ and HD molecular ions have because of their physical properties been used as test ions for testing the new electron cooler and comparing different ion storage rings. Dissociative recombination cross sections and branching ratios of XH_n⁺ type ions have been determined. The dissociative recombination and excitation cross sections of O₂⁺ were measured and the mechanism involved in the green airglow was investigated by an imaging technique. Also, for the first time, resonant ion-pair formation mechanisms of HD⁺, HF⁺ and OH⁺ were investigated using the ion storage ring.