Dissociative recombination of N2_2H+^+: A revisited study

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 tabl

Population of ground and lowest excited states of Sulfur via the dissociative recombination of SH+ in the diffuse interstellar medium

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 table

Ab initio calculations of autoionizing states using block diagonalization: Collinear diabatic states for dissociative recombination of electrons with N2H+

International audienceDissociating autoionizing states for dissociative recombination of electrons with N2H+ have been calculated using block diagonalization. Multi-reference CI calculations for collinear N2H and N2H+ were performed to assess the branching ratio to the product channels. The effects of the strong Rydberg-valence mixing in the N2H excited states were disentangled from the changes in the molecular orbitals arising solely from N-2 bond stretching and breaking. The results suggest that N-2 + H should be favored over NH + N, because of the absence of a favorable dissociating state for the N-2 bond breaking

How to obtain accurate diabatic surfaces governing the dissociative recombination of astrophysical ions

International audienceElectronic dissociative recombination (DR) AB+ + e− → A + B processes are important for astrophysical, combustion and fusion plasmas. In the interstellar medium they determine the abundance of the positively charged species and knowing their efficiency is of primary importance. Their theoretical study requires two steps: (i) electronic structure calculations to obtain potential energy surfaces and corresponding coupling terms, and (ii) collision dynamic treatments using the potentials and the coupling terms obtained from the previous step, to calculate rates constants and branching ratio. The present work details the first step.The states involved in the DR of an ion are of different types, namely, the ionic, Rydberg and dissociating states of the corresponding neutral. Calculating them is not routine since their nature changes along the dissociating channels and carefully designed wavefunctions are needed to follow those transformations. Moreover, complications rise for the dissociating states that are embedded in the continuum of scattering states that correspond to AB+ + e-, because they are highly excited and strongly mixed with other states. In a DR process multiple curve crossings occur, and it can be very difficult to isolate the desired Rydberg and dissociating potential surfaces. In order to insure their quantitative treatment, crucial for accurate rate constants calculations though step (ii) we have developed over the past few years [1-3] a methodology that uses the block diagonalization method [4] to determine accurate diabatic surfaces from the MRCI adiabatic ones as well as the corresponding electronic couplings which are needed for the collision dynamic treatment (step ii). The power of this methodology is outlined through our study of the DR of HCNH+, N2H+ and SH+

A theoretical study of the dissociative recombination of SH + with electrons through the 2 Π states of SH

International audienceA 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 stepwise 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 ion Test Storage Ring (TSR) in Heidelberg, Germany