Dissociative recombination of CH+: Cross section and final states

The cross section as well as the branching ratios for the dissociative recombination of ground-state CH+ ions with electrons have been measured using the heavy-ion storage-ring technique and two-dimensional fragment imaging. Although the absolute value of the cross section at thermal energies is found to be in very good agreement with the theory, several unpredicted narrow resonances are also present in the data. These structures are interpreted as due to an indirect recombination process via core-excited Rydberg states. The branching-ratio measurement shows that at low electron energy the 2 (2) Pi state, producing carbon fragments C(D-1), is the most important dissociative state, although transitions during the dissociation to other dissociative potential curves are also present. Anisotropy in the angular distribution of the dissociating fragments is visible for some of the final states. Dissociative recombination of ions in the metastable excited a (3) Pi state is also observed, and the lifetime as well as the excitation energy of this state are deduced from the imaging data.