Annick Suzor-Weiner

Dissociative recombination of H3+: progress in theory

Dissociative recombination is the main destruction process for ground–state H3+ in diffuse interstellar medium. Experiments agree on relatively large cross–sections for this reaction. Time–dependent two–dimensional calculations confirm the experimental results at high energy as well as the observed predissociation rates of H3 Rydberg states, due to non–adiabatic interactions. However, the value for low–energy crosssection, deduced from the predissociation rates by an extrapolation procedure, is about four orders of magnitude lower than the measured one. A calculation based on multichannel quantum defect theory suggests that an indirect non–adiabatic process may prevail in this case. The cross–section increases by orders of magnitude compared with the extrapolated value when indirect couplings via apparently ineffective channels are properly considered. We discuss how this channel–mixing mechanism can be effective in the case of H3+, and show encouraging results stressing the role of Rydberg series or ‘closed channels’. We also discuss possible three–dimensional effects that could enhance the process at low energy.

Chemistry. Mystery of an interstellar ion.

Hydrogen is the most abundant element in the Universe. Its chemistry governs most reactions in space, especially when it is ionized. But the abundance of the H3+ ion is not easy to explain.

H2 triplet states contribution to low-energy dissociative recombination of H2+

Motivated by the observation of the predissociation of H2 triplet states in low vibrational levels, we evaluate the contribution of triplet dissociative states to the low-energy dissociative recombination of H2+ in vibrational levels where v = 0, 1. The triplet states contribute mainly via radial couplings between monoexcited Rydberg configurations without curve crossings. The total contribution of the six triplet states leading to dissociation in H(n = 1) + H(n = 2) is found to be smaller by at least one order of magnitude than the contribution of the lowest doubly excited singlet state, (2pσu)2 1 Σ g+.

Theoretical study of a cold atom beam splitter

A theoretical model is presented for the study of the dynamics of a cold atomic cloud falling in the gravity field in the presence of two crossing dipole guides. The cloud is split between the two branches of this laser guide, and we compare experimental measurements of the splitting efficiency with semiclassical simulations. We then explore the possibilities of optimization of this beam splitter. Our numerical study also gives access to detailed information, such as the atom temperature after the splitting.

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.

Ab initio study of the dissociative recombination of Ne2

We report extensive calculations of energy positions and autoionization widths for the doubly excited states of Ne2+ between the first and second ionization thresholds obtained from electron scattering calculations using the complex Kohn variational method. The dynamics of the dissociative recombination process was investigated using multichannel quantum defect theory. For these preliminary calculations, only the direct mechanism of the dissociative recombination reaction was studied and only the 1,3Σg+ dissociative states, which lie closest in energy to the ion at its internuclear separation were included. These states should dominate the low-energy dissociative recombination.