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A quantitative theoretical study of the dissociative recombination of SH$^+$ with electrons has been carried out. Multireference, configuration interaction calculations were used to determine accurate potential energy curves for SH$^+$ and SH. The block diagonalization method was used to disentangle strongly interacting SH valence and Rydberg states and to construct a diabatic Hamiltonian whose diagonal matrix elements provide the diabatic potential energy curves. The off-diagonal elements are related to the electronic valence-Rydberg couplings. Cross sections and rate coefficients for the dissociative recombination reaction were calculated with a step-wise version of the multichannel quantum defect theory, using the molecular data provided by the block diagonalization method. The calculated rates are compared with the most recent measurements performed on the TSR ion storage ring in Heidelberg, Germany., Comment: 17 pages, 15 figures, 4 tables

Multichannel quantum defect theory is applied in the treatment of the dissociative recombination and vibrational excitation processes for the BeD$^+$ ion in the twenty four vibrational levels of its ground electronic state ($\textrm{X}\,{^{1}\Sigma^{+}},v_{i}^{+}=0\ldots 23$). Three electronic symmetries of BeD$^{**}$ states (\ensuremath{^{2}\Pi}, \ensuremath{^{2}\Sigma^{+}}, and \ensuremath{^{2}\Delta}), are considered in the calculation of cross sections and the corresponding rate coefficients. The incident electron energy range is $10^{-5}$--2.7 eV and the electron temperature range is 100--5000~K. The vibrational dependence of these collisional processes is highlighted. The resulting data are useful in magnetic confinement fusion edge plasma modelling and spectroscopy, in devices with beryllium based main chamber materials, such as ITER and JET, and operating with the deuterium-tritium fuel mix. An extensive rate coefficients database is presented in graphical form and also by analytic fit functions whose parameters are tabulated in the supplementary material., Comment: 7 pages, 5 figures, 3 tables

Calculations are performed for electron collision with the methylene molecular ion CH$_2^+$ in its bent equilibrium geometry, with the goal to obtain cross sections for electron impact excitation and dissociation. The polyatomic version of the UK molecular R-matrix codes was used to perform an initial configuration-interaction calculation on the doublet and quartet states of the CH$_2^+$ ion. Subsequently, scattering calculations are performed to obtain electron impact electronic excitation and dissociation cross sections and, additionally, the bound states of the CH$_2$ molecule and Feshbach resonances in the $e$-CH$_2^+$ system., Comment: 6 pages, 6 figures and 3 tables

A series of Multichannel Quantum Defect Theory-based computations have been performed, in order to produce the cross sections of rotational transitions (excitations $N_{i}^{+}-2 \rightarrow$ $N_{i}^{+}$, de-excitations $N_{i}^{+}$ $\rightarrow$ $N_{i}^{+}-2$, with $N_{i}^{+}=2$ to $10$) and of their competitive process, the dissociative recombination, induced by collisions of HD$^+$ ions with electrons in the energy range $10^{-5}$ to 0.3 eV. Maxwell anisotropic rate coefficients, obtained from these cross sections in the conditions of the Heidelberg Test Storage Ring (TSR) experiments ($k_{B}T_{t}=2.8$ meV and $k_{B}T_{l}=45$ $\mu$eV), have been reported for those processes in the same electronic energy range. Maxwell isotropic rate coefficients have been as well presented for electronic temperatures up to a few hundreds of Kelvins. Very good overall agreement is found between our results for rotational transitions and the former theoretical computations as well as with experiment. Furthermore, owing to the full rotational computations performed, the accuracy of the resulting dissociative recombination cross sections is considerably improved., Comment: 9 pages, 8 figures, 1 table. arXiv admin note: text overlap with arXiv:1107.5267

The latest molecular data - potential energy curves and Rydberg$/$valence interactions - characterizing the super-excited electronic states of CO are reviewed, in order to provide inputs for the study of their fragmentation dynamics. Starting from this input, the main paths and mechanisms for CO$^+$ dissociative recombination are analyzed; its cross sections are computed using a method based on Multichannel Quantum Defect Theory. Convoluted cross sections, giving both isotropic and anisotropic Maxwellian rate-coefficients, are compared with merged-beam and storage-ring experimental results. The calculated cross sections underestimate the measured ones by a factor of $2$, but display a very similar resonant shape. These facts confirm the quality of our approach for the dynamics, and call for more accurate and more extensive molecular structure calculations., Comment: 12 pages, 8 figures 5 tables

The latest molecular data - potential energy curves and Rydberg-valence interactions - characterising the super-excited electronic states of BF are reviewed in order to provide the input for the study of their fragmentation dynamics. Starting from this input, the main paths and mechanisms of BF$^+$ dissociative recombination and vibrational excitation are analysed. Their cross sections are computed for the first time using a method based on the multichannel quantum defect theory (MQDT), and Maxwellian rate-coefficients are calculated and displayed in ready-to-be-used format for low temperature plasma kinetics simulations., Comment: 8 pages, 4 figures, 4 tables

Cross-sections for dissociative recombination and electron-impact vibrational excitation of the BF$^+_2$ molecular ion are computed using a theoretical approach that combines the normal modes approximation for the vibrational states of the target ion and use of the UK R-matrix code to evaluate electron-ion scattering matrices for fixed geometries of the ion. Thermally-averaged rate coefficients are obtained from the cross-sections for temperatures in the 10-3000 K range., Comment: 6 pages, 5 figures, 2 tables

The reactive collisions of the CH$^+$ molecular ion with electrons is studied in the framework of the multichannel quantum defect theory, taking into account the contribution of the core-excited Rydberg states. In addition to the $X^1\Sigma^+$ ground state of the ion, we also consider the contribution to the dynamics of the $a^3\Pi$ and $A^1\Pi$ excited states of CH$^+$. Our results - in the case of the dissociative recombination in good agreement with the storage ring measurements - rely on decisive improvements - complete account of the ionisation channels and accurate evaluation of the reaction matrix - of a previously used model., Comment: 8 pages, 8 figures

The bound and resonance states along with corresponding autoionization widths for nitrogen sulphide (NS) molecule are determined using electron NS$^+$ cation scattering calculations. The calculations are performed for $^2{\Sigma}^+$, $^2{\Pi}$ and $^2\Delta$ total symmetries using the ab initio R-matrix method for both bound and continuum states. Calculations are performed on a grid of 106 points for internuclear separations between 1.32 and 3 $\AA$. The resonance states yield dissociative potential curves which, when considered together with their widths, provide input for models of different electron-cation collision processes including dissociative recombination, and rotational and vibrational excitation. Curves and couplings which will lead directly to dissociative recombination are identified., Comment: 7 pages, 11 figures

Cross sections and rate coefficients for the Dissociative Recombination (DR) of the NS+ ion induced by collisions with low-energy electrons are reported for temperatures between 10 and 1000 K, relevant to a large range of interstellar cloud temperatures. Uncertainties are discussed for these rates. Comparisons are made with DR rates for the isovalent NO+ molecular ion which are found to be much faster. The present findings lead to a moderate dissociative reaction rate coefficient, smaller by a factor of 2 than the current estimates reported in the different kinetic databases for a temperature of 10 K. We consider that our rate coefficients obtained through multichannel quantum defect theory for NS+ are likely to be better than those displayed in the different kinetic databases., Comment: 6 pages, 8 figures, 2 tables

Our previous studies of dissociative recombination, and vibrational excitation/de-excitation of the BeH$^+$ ion, based on the multichannel quantum defect theory, are extended to collision energies above the dissociation threshold, taking into account the vibrational continua of the BeH$^+$ ion and, consequently, its dissociative excitation. We have also significantly increased the number of dissociative states of $^2{\Pi}$, $^2{\Sigma}^+$ and $^2\Delta$ symmetry included in our cross section calculations, generating the most excited-ones by using appropriate scaling laws. Our results are suitable for modeling the kinetics of BeH+ in edge fusion plasmas for collision energies up to 12 eV., Comment: 9 pages, 9 figures

Dissociative recombination of N$_2$H$^+$ is explored in a two-step theoretical study. In a first step, a diatomic (1D) rough model with frozen NN bond and frozen angles is adopted, in the framework of the multichannel quantum defect theory (MQDT). The importance of the indirect mechanism and of the bending mode is revealed, in spite of the disagreement between our cross section and the experimental one. In a second step, we use our recently elaborated 3D approach based on the normal mode approximation combined with R-matrix theory and MQDT. This approach results in satisfactory agreement with storage-ring measurements, significantly better at very low energy than the former calculations., Comment: 9 pages, 5 figures, 1 table

Our previous study on dissociative recombination of ground state SH$^+$ into $^2\Pi$ states of SH is extended by taking into account the contribution of $^4\Pi$ states recently explored by quantum chemistry methods. Multichannel quantum defect theory is employed for the computation of cross sections and rate coefficients for dissociative recombination, but also for vibrational excitation. Furthermore, we produce the atomic yields resulting from recombination, quantifying the generation of sulfur atoms in their ground (\mbox{$^3$P}) and lowest excited (\mbox{$^1$D}) states respectively., Comment: 9 pages, 8 figures, 3 tables

We review the study of dissociative recombination and ro-vibrational excitation of diatomic and small polyatomic molecular ions initiating complex organic molecules formation. In particular, we show how Multichannel Quantum Defect Theory (MQDT) and R-matrix methods are used to compute cross sections and rate coefficients for cations in well defined ro-vibrational levels of the ground electronic state, from sub-meV up to few eV collision energies. The most recent MQDT results are compared with either other theoretical data, or with measured data obtained in storage-ring experiments., Comment: 20 pages, 13 figures

We used the multichannel quantum defect theory to compute cross sections and rate coefficients for the dissociative recombination of CH$^+$ initially in its lowest vibrational level $v_i^+ = 0$ with electrons of incident energy bellow $0.2$ eV. We have focused on the contribution of the $2$ $^2\Pi$ state which is the main dissociative recombination route at low collision energies. The final cross section is obtained by averaging the relevant initial rotational states $(N_i^+ = 0,\dots,10)$ with a $300$ K Boltzmann distribution.The Maxwell isotropic rate coefficients for dissociative recombination are also calculated for different initial rotational states and for electronic temperatures up to a few hundred Kelvins. Our results are compared to storage-ring measurements., Comment: 5 pages, 5 figures

Rate coefficients for the dissociative recombination, vibrational excitation and vibrational de-excitation of the BeT$^{+}$ ion for all vibrational levels of its ground electronic state ($ X\ensuremath{^{1}\Sigma^{+}},v_{i}^{+}=0,\dots,27$) are reported, including in the calculation the contribution of super-excited states of the BeT complex pertaining to three electronic symmetries - $^{2}\Pi$, $^{2}\Sigma^{+}$, and $^{2}\Delta$. These data are suitable for the kinetic modeling of beryllium and tritium containing plasma, as encountered in magnetic fusion devices with beryllium walls (JET, ITER). In the present study we restrict ourselves to incident electron energies from 10$^{-3}$ up to $2.7$ eV, and to electron temperatures between $100$ and $5000$ K, respectively. Together with our earlier and closely related studies on the BeH$^{+}$ and BeD$^{+}$ systems, this present work completes the isotopic coverage for the beryllium monohydride ions. The vibrational energy (rather than the vibrational quantum state) is identified as a proper isotopic similarity parameter, e.g., for reduced but still isotopically correct plasma chemistry models., Comment: 27 pages, 7 figures, 15 tables

We describe the major low-energy electron-impact processes involving H$^+_2$ and HD$^+$, relevant for the astrochemistry of the early Universe: Dissociative recombination, elastic, inelastic and superelastic scattering. We report cross sections and Maxwellian rate coefficients of both rotational and vibrational transitions, and outline several important features, like isotopic, rotational and resonant effects., Comment: 5 pages, 4 figures

Cross sections and thermal rate coefficients are computed for electron-impact dissociative recombination and vibrational excitation/de-excitation of the N$^+_2$ molecular ion in its lowest six vibrational levels, for collision energies/temperatures up to 2.3 eV/5000 K., Comment: 9 pages, 4 figures, 3 tables

Rate coefficients for dissociative recombination and state-to-state rotational transitions of the D$_{2}^{+}$ ion induced by collisions with very low-energy electrons have been reported following our previous studies on HD$^{+}$ and H$_{2}^{+}$ [9,10]. The same molecular structure data sets, excitations ($N_{i}^{+} \rightarrow$ $N_{f}^{+}=N_{i}^{+}+2$ for $N_{i}^{+}=0$ to $10$) and de-excitations ($N_{i}^{+}$ $\rightarrow$ $N_{f}^{+}=N_{i}^{+}-2$, for $N_{i}^{+}=2$ to $10$) were used for collision energies ranging from $0.01$ meV to $0.3$ eV. Isotopic effects for dissociative recombination and rotational transitions of the vibrationally relaxed targets are presented., Comment: 7 pages, 7 figures, 4 tables

Motivated by the huge need of data for non-equilibrium plasma modeling, a theoretical investigation of dissociative electron attachment to the NO molecule is performed. The calculations presented here are based on the Local-Complex-Potential approach, taking into account five NO$^-$ resonances. Three specific channels of the process are studied, including the production of excited nitrogen atoms $\mathrm{N}(^2\mathrm{D})$ and of its anions N$^-$. Interpretation of the existing experimental data and their comparison with our theoretical result are given. A full set of ro-vibrationally-resolved cross sections and the corresponding rate coefficients are reported. In particular, a relatively notably large cross section of N$^-$ ion formation at low energy of the incident electron and for vibrationally excited NO target is predicted. Finally, molecular rotation effects are discussed., Comment: 7 pages, 7 figures

A theoretical investigation of the dissociative excitation by electron impact on the NO molecule is presented, aiming to make up for the lack of data for this process in the literature. A full set of vibrationally-resolved cross sections and corresponding rate coefficients are calculated using the Local-Complex-Potential approach and five resonant states of NO^-., Comment: 5 pages, 3 figures

Cross sections are presented for dissociative recombination and electron-impact vibrational excitation of the ArH+ molecular ion at electron energies appropriate for the interstellar environment. The R-matrix method is employed to determine the molecular structure data, i.e. the position and width of the resonance states. The cross sections and the corresponding Maxwellian rate coefficients are computed using a method based on the Multichannel Quantum Defect Theory. The main result of the paper is the very low dissociative recombination rate found at temperatures below 1000K. This is in agreement with the previous upper limit measurement in merged beams and offers a realistic explanation to the presence of ArH+ in exotic interstellar conditions., Comment: 6 pages; 7 figures

In order to improve our understanding of the kinetics of the cometary coma, theoretical studies of the major reactive collisions in these environments are needed. Deep in the collisional coma, inelastic collisions between thermal electrons and molecular ions result in recombination and vibrational excitation, the rates of these processes being particularly elevated due to the high charged particle densities in the inner region. This work addresses the dissociative recombination, vibrational excitation, and vibrational de-excitation of electrons with CO$^+$ molecular cations. The aim of this study is to understand the importance of these reactive collisions in producing carbon and oxygen atoms in cometary activity. The cross-section calculations were based on Multichannel Quantum Defect Theory. The molecular data sets, used here to take into account the nuclear dynamics, were based on ab initio R-matrix approach. The cross sections for the dissociative recombination, vibrational excitation, and vibrational de-excitation processes, for the six lowest vibrational levels of CO$^+$ - relevant for the electronic temperatures observed in comets - are computed, as well as their corresponding Maxwell rate coefficients. Moreover, final state distributions for different dissociation pathways are presented. Among all reactive collisions taking place between low-energy electrons and CO$^+$, the dissociative recombination is the most important process at electronic temperatures characterizing the comets. We have shown that this process can be a major source of O($^3$P), O($^1$D), O($^1$S), C($^3$P) and C($^1$D) produced in the cometary coma at small cometocentric distances., Comment: 8 pages, 6 figures, regular paper

We present a detailed theoretical study of the rotational excitation of CH$^+$ due to reactive and nonreactive collisions involving C$^+(^2P)$, H$_2$, CH$^+$, H and free electrons. Specifically, the formation of CH$^+$ proceeds through the reaction between C$^+(^2P)$ and H$_2(\nu_{\rm H_2}=1, 2)$, while the collisional (de)excitation and destruction of CH$^+$ is due to collisions with hydrogen atoms and free electrons. State-to-state and initial-state-specific rate coefficients are computed in the kinetic temperature range 10-3000~K for the inelastic, exchange, abstraction and dissociative recombination processes using accurate potential energy surfaces and the best scattering methods. Good agreement, within a factor of 2, is found between the experimental and theoretical thermal rate coefficients, except for the reaction of CH$^+$ with H atoms at kinetic temperatures below 50~K. The full set of collisional and chemical data are then implemented in a radiative transfer model. Our Non-LTE calculations confirm that the formation pumping due to vibrationally excited H$_2$ has a substantial effect on the excitation of CH$^+$ in photon-dominated regions. In addition, we are able to reproduce, within error bars, the far-infrared observations of CH$^+$ toward the Orion Bar and the planetary nebula NGC~7027. Our results further suggest that the population of $\nu_{\rm H_2}=2$ might be significant in the photon-dominated region of NGC~7027., Comment: 10 pages, 8 figures, accepted 2017 April 7

We provide cross sections and Maxwell rate coefficients for reactive collisions of slow electrons with BeH$^+$ ions on all the eighteen vibrational levels ($X{^{1}\Sigma^{+}},v_{i}^{+}=0,1,2,\dots,17$) using a Multichannel Quantum Defect Theory (MQDT) - type approach. These data on dissociative recombination, vibrational excitation and vibrational de-excitation are relevant for magnetic confinement fusion edge plasma modelling and spectroscopy, in devices with beryllium based main chamber materials, such as the International Thermonuclear Experimental Reactor (ITER) and the Joint European Torus (JET). Our results are presented in graphical form and as fitted analytical functions, the parameters of which are organized in tables., Comment: Atomic Data and Nuclear Data Tables, in press

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)

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)