Dissociative recombination of vibrationally excited HD+: State-selective experimental investigation

The relative dissociative recombination rate coefficients for specific vibr ational states of HD+ have been measured. The method is based on using merg ed electron and molecular ion beams in a heavy-ion storage ring together wi th molecular fragment imaging techniques which allow us to probe the vibrat ional-state population of the stored beam as a function of time as well as the final state of the dissociation. The initial vibrational distribution o f the stored ion beam (from a Penning ion source) is found to be in good ag reement with a Franck-Condon model of electron impact ionization, apart fro m slightly larger experimental populations found for low vibrational states ; its time evolution in the storage ring reflects the predicted vibrational level lifetimes. Dissociative recombination measurements were performed wi th the electron and ion beams at matched velocities (corresponding to avera ge collision energies of about 10 meV), and at several well-defined collisi on energies in the range of 3-11 eV. The obtained vibrational-state specifi c recombination rate coefficients are compared with theoretical calculation s and show that, although an overall agreement exists between experiment an d theory, large discrepancies occur for certain vibrational states at low e lectron energy. [S1050-2947(99)09411-1].