Anisotropic fragmentation in low-energy dissociative recombination

On a dense energy grid reaching up to 75 meV electron collision energy the fragmentation angle and the kinetic energy release of neutral dissociative recombination fragments have been studied in a twin merged beam experiment. The anisotropy described by Legendre polynomials and the extracted rotational state contributions were found to vary on a likewise narrow energy scale as the rotationally averaged rate coefficient. For the first time angular dependences higher than 2$^{nd}$ order could be deduced. Moreover, a slight anisotropy at zero collision energy was observed which is caused by the flattened velocity distribution of the electron beam.Comment: 8 pages, 4 figures; The Article will be published in the proceedings of DR 2007, a symposium on Dissociative Recombination held in Ameland, The Netherlands (18.-23. July 2008); Reference 19 has been published meanwhile in S. Novotny, PRL 100, 193201 (2008

Assignment of resonances in dissociative recombination of HD+ ions: high-resolution measurements compared with accurate computations

The collision-energy resolved rate coefficient for dissociative recombination of HD+ ions in the vibrational ground state is measured using the photocathode electron target at the heavy-ion storage ring TSR. Rydberg resonances associated with ro-vibrational excitation of the HD+ core are scanned as a function of the electron collision energy with an instrumental broadening below 1 meV in the low-energy limit. The measurement is compared to calculations using multichannel quantum defect theory, accounting for rotational structure and interactions and considering the six lowest rotational energy levels as initial ionic states. Using thermal equilibrium level populations at 300 K to approximate the experimental conditions, close correspondence between calculated and measured structures is found up to the first vibrational excitation threshold of the cations near 0.24 eV. Detailed assignments, including naturally broadened and overlapping Rydberg resonances, are performed for all structures up to 0.024 eV. Resonances from purely rotational excitation of the ion core are found to have similar strengths as those involving vibrational excitation. A dominant low-energy resonance is assigned to contributions from excited rotational states only. The results indicate strong modifications in the energy dependence of the dissociative recombination rate coefficient through the rotational excitation of the parent ions, and underline the need for studies with rotationally cold species to obtain results reflecting low-temperature ionized media.Comment: 15 pages, 10 figures. Paper to appear in Phys. Rev. A (version as accepted

Towards a model for the dissociative recombination of the CO2+dication: states and couplings

Preliminary results which will be used for calculations of dissociative recombination (DR) of electrons with the CO2+ dication are presented. The measurements of CO2+ dissociative recombination rates, the first for any multiply charged target, were obtained at the ASTRID heavy ion storage ring. The present study starts from the potential energy curves for the first 7 electronic states of CO2+ and results for low-energy e− – CO2+ scattering that were obtained in recent R-matrix calculations (Vinci and Tennyson 2004 J. Phys. B 37 2011). Meanwhile, in order to apply an MQDT-type approach that has been previously used for NO+, we concentrate on partial the resonance series converging to the 1Δ target state in the CO+ 2Σ+ symmetry. The quasi-discrete vibrational spectrum of the CO2+ ground electronic state is explored

Reactive collisions between electrons and NO+ ions: rate coefficient computations and relevance for the air plasma kinetics

International audienceExtensive calculations of the rate coefficients for dissociative recombination (DR), elastic collisions, inelastic collisions (ICs) and superelastic collisions of NO+ ions on initial vibrational levels, v_{\rmi}^{+}=0\hbox{--}14 , with electrons of energy between 10−5 and 10 eV have been performed, with a method based on multichannel quantum defect theory. Comparisons of the DR rate coefficients with the plasma experimental results give a good agreement, confirming that the vibrationally excited NO+ ions recombine more slowly than those in the ground state. Also, our ground state IC rate coefficients are very similar to previously computed R-matrix data. The rate coefficients have been fitted to a modified Arrhenius law, and the corresponding parameters are given, in order to facilitate the use of the reaction data in kinetical plasma modelling

Lázár, Gyula

(1841 - 1912), Historike

Reactive Collisions Between Electrons and Molecular Hydrogen Cation Isotopomers: Cross-sections and Rate Coefficients for HD+ and DT+

International audienceCross sections for dissociative recombination, dissociative excitation, elastic, inelastic and superelastic collisions of HD+ andDT+ on ground and first three excited vibrational levels, with electrons of energy between 0.01 meV and 13 eV, are computedwithin a method based on the multichannel quantum defect theory (MQDT). These cross sections, together with the corre-sponding Maxwell isotropic rate coefficients for electronic temperature between 100 K and 40 000 K, aim at providing inputdata for the kinetic modelling of edge fusion plasmas and other hydrogen containing plasmas

Assignment of resonances in dissociative recombination of HD⁺ ions: high-resolution measurements compared with accurate computations

The collision-energy resolved rate coefficient for dissociative recombination of HD+ ions in the vibrational ground state is measured using the photocathode electron target at the heavy-ion storage ring TSR. Rydberg resonances associated with ro-vibrational excitation of the HD+ core are scanned as a function of the electron collision energy with an instrumental broadening below 1 meV in the low-energy limit. The measurement is compared to calculations using multichannel quantum defect theory, accounting for rotational structure and interactions and considering the six lowest rotational energy levels as initial ionic states. Using thermal equilibrium level populations at 300 K to approximate the experimental conditions, close correspondence between calculated and measured structures is found up to the first vibrational excitation threshold of the cations near 0.24 eV. Detailed assignments, including naturally broadened and overlapping Rydberg resonances, are performed for all structures up to 0.024 eV. Resonances from purely rotational excitation of the ion core are found to have similar strengths as those involving vibrational excitation. A dominant low-energy resonance is assigned to contributions from excited rotational states only. The results indicate strong modifications in the energy dependence of the dissociative recombination rate coefficient through the rotational excitation of the parent ions, and underline the need for studies with rotationally cold species to obtain results reflecting low-temperature ionized media

Rotational Cooling of HD+ by Superelastic Collisions with Electrons

International audienceRotational cooling of HD+ by superelastic collisions (SEC) with electrons was observed at the Heidelberg test storage ring by merging a beam of rotationally hot HD+ ions with an electron beam at zero relative energy. Neutral fragments resulting from DR events were recorded at different electron densities using a high resolution imaging detector and a large-area, energy sensitive detector. The data allowed to deduce the time dependence of the population of three groups of rotational angular momentum states J built on the vibrational ground state of the ion together with the corresponding DR rate coefficients. The latter are found to be (statistical uncertainties only) langlealpharangle0,1,2 = 3.8(1), langlealpharangle3,4 = 4.0(2), and langlealpharangle5,6,7 = 9.0(1.3) in units of 10-8 cm3/s, in reasonable agreement with the average values derived within the MQDT approach. The time evolution of the population curves clearly reveals that rotational cooling by SEC takes place, which can be well described by using theoretical SEC rate coefficients obtained by combining the molecular R-matrix approach with the adiabatic nuclear rotation approximation. We verify the DeltaJ = -2 coefficients, which are predicted to be dominant as opposed to the DeltaJ = -1 coefficients and to amount to (1 - 2) · 10-6 cm3/s, to within 30%