The role of Rydberg states in dissociative recombination, as revealed by ion storage ring experiments

After a brief presentation of the dissociative recombination process and of the recent measurements performed with ion storage rings by several groups of experimentalists, we analyse the role of Rydberg states in the dynamics of the reaction. They are responsible for various kinds of resonances observed in the experimental cross sections. Bound Rydberg states cause narrow resonances, mostly in the form of dips at very low energy (rovibrationally excited Rydberg states with ground state ion core), and of peaks at higher energy (Rydberg states with excited state ion core). At still higher energy (approximately 10 eV and higher) entire series of dissociative Rydberg states with repulsive ion core cause very broad (a few eV) composite resonances which dominate the process. Competition with dissociative excitation (AB+ + e → A + B+ + e) must be included in the theoretical treatment above the ion dissociation threshold. Calculations for HD+ and CD+, based on a multichannel quantum defect approach, are described.