Ioan F. Schneider

Collisional-radiative model in air for earth re-entry problems

A nonlinear time-dependent two-temperature collisional-radiative model for air plasma has been developed for pressures between 1kPa and atmospheric pressure to be applied to the flow conditions of space vehicle re-entry into the Earth’s atmosphere. The model consists of 13 species: N2, O2, N, O, NO, N2+, O2+, N+, O+, NO+, O2−, O− in their ground state and major electronic excited states and of electrons. Many elementary processes are considered given the temperatures involved (up to 10 000K). Time scales to reach the final nonequilibrium or equilibrium steady states are derived. Then we apply our model to two typical re-entry situations and show that O2− and O− play an important role during the ionization phase. Finally, a comparison with existing reduced kinetic mechanisms puts forward significant discrepancies for high velocity flows when the flow is in chemical nonequilibrium and smaller discrepancies when the flow is close to chemical equilibrium. This comparison illustrates the interest of using a ti...

Dissociative recombination of NO + : calculations and comparison with experiment

Multichannel quantum defect calculations for NO + dissociative recombination (DR) for electron energies from threshold to 8 eV are presented. The calculations use electronic energies and autoionization widths of valence states obtained from ab initio R-matrix calculations with the corresponding potential curves calibrated using available spectroscopic data. Six valence states open to dissociation are included in the final calculations. Excellent agreement with the measured cross sections is obtained for the low-energy DR, up to 3 eV and, for the first time, the peak observed in the cross section at high energy is accounted for. The importance of the various dissociative states at different electron energies, as well as the direct and indirect processes, is discussed. Compared to previous theoretical studies, the inclusion of a third dissociative state of 2 � symmetry and the larger autoionization width of the B� 2� state are found to be particularly important for the agreement with experiment. (Some figures in this article are in colour only in the electronic version; see www.iop.org)

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+.

Imaging spectroscopy of recombination fragments of

Dissociative recombination of vibrationally cold has been studied at the ion storage ring CRYRING, with an accurate measurement of the kinetic energy of the recombination fragments. When the incident electron energy exceeds , fragments are observed with a kinetic energy of only . The energetics suggest that this low-energy channel derives from either recombination of the state or recombination of the metastable state. High-quality ab initio calculations have been used to obtain the state vibrational relaxation time (10 ms), excitation energy (2.25 eV) and radiative lifetime (0.030 s). The dissociative recombination cross section has been obtained in a model calculation. The results imply that the state has too small a recombination cross section and too short a radiative lifetime to explain the experimental results, which were obtained after 15 s of storage in CRYRING. We conclude that the 120 meV channel is due to recombination of into the asymptotic limit. Possible mechanisms are discussed.

Experimental characterization of the excitation state of picosecond laser-induced Tungsten plasmas

In order to quantitatively determine the retention of light atoms by plasma facing components (mainly tungsten) used in fusion reactors, the Laser-Induced Breakdown Spectroscopy (LIBS) technique can be used. The nanosecond laser regimes classically used for LIBS present major limitations mainly due to the conditions of the laser-matter interaction process itself and cannot be used in the present context. Picosecond laser pulses are more appropriate. To validate the diagnostic based on picosecond laser pulses, preliminary characterizations of the produced plasmas have to be performed. Some related results are reported in the present communication. The study is focused on (1) the ablation of tungsten under two laser (low and high) fluence conditions and (2) the excitation equilibrium of the plasma from the nanosecond to microsecond time scales.

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.