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

Diabatic potential energy curves for the 4Π^4{\Pi} states of SH

International audienceAbstract We present a diabatic representation of the potential energy curves (PECs) for the $^4{{\Pi}}$ 4 Π states of $\mathrm {SH}$ SH . Multireference, configuration interaction (MRCI) calculations were used to determine high-accuracy adiabatic PECs of both $\mathrm {SH}$ SH and ${\mathrm {SH}}^+$ SH + from which the diabatic representation is constructed for $\mathrm {SH}$ SH . The adiabatic PECs exhibit many avoided crossings due to strong Rydberg-valence mixing. We employ the block diagonalization method, an orthonormal rotation of the adiabatic Hamiltonian, to disentangle the valence autoionizing and Rydberg $^4\Pi$ 4 Π states of $\mathrm {SH}$ SH by constructing a diabatic Hamiltonian. The diagonal elements of the diabatic Hamiltonian matrix at each nuclear geometry render the diabatic PECs and the off-diagonal elements are related to the state-to-state coupling. Care is taken to assure smooth variation and consistency of chemically significant molecular orbitals across the entire geometry domain

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+

Using block diagonalization to determine dissociating autoionizing states: Application to N2H, and the outlook for SH

International audienceWe describe our implementation of the block diagonalization method for calculating the potential surfaces necessary to treat dissociative recombination (DR) of electrons with N2H+. Using the methodology we have developed over the past few years, we performed multi-reference, configuration interaction calculations for N2H+ and N2H with a large active space using the GAMESS electronic structure code. We treated both linear and bent geometries of the molecules, with N2 fixed at its equilibrium separation. Because of the strong Rydberg-valence coupling in N2H, it is essential to isolate the appropriate dissociating, autoionizing states. Our procedure requires only modest additional effort beyond the standard methodology. The results indicate that the crossing between the dissociating neutral curve and the initial ion potential is not favorably located for DR, even if the molecule bends. The present calculations thereby confirm our earlier results for linear N2H and reinforce the conclusion that the direct mechanism for DR is likely to be inefficient. We also describe interesting features of our preliminary calculations on SH

Calculation of dissociating autoionizing states using the block diagonalization method: Application to N2H

We report the calculation of preliminary potential surfaces necessary to treat dissociative recombination (DR) of electrons with N2H+. We performed multi-reference, configuration interaction calculations with a large active space for N2H+ and N2H, using the GAMESS electronic structure code. Rydberg-valence coupling is strong in N2H, and a systematic procedure is desirable to isolate the appropriate dissociating, autoionizing states. We used the block diagonalization method, which requires only modest additional effort beyond the standard methodology. We treated both linear and bent geometries of the molecules, with N2 fixed at its equilibrium separation. The results indicate that the crossing between the dissociating neutral curve and the initial ion potential is not favorably located, suggesting that the direct mechanism for DR will be small. Dynamics calculations using the multi-configuration, time-dependent Hartree (MCTDH) method confirm this conclusion

A comparative study of the DR reactions of c-C3H+3 and l-C3H+3: Preliminary theoretical studies

International audiencePreliminary calculations related to the dissociative recombination (DR) of electrons with C3H+3 have been carried out. Both the linear and cyclic isomers of this ion exist in the interstellar medium, and accurate DR rate constants for both isomers are needed for astrophysical models. The electronic structure calculations reported here yield quasi-diabatic potential energy curves that can be used to assess the efficiency of dissociation of a CH bond. The calculations confirm a favorable position of a dissociative state for the cyclic isomer and suggest that dissociation of the linear isomer is less probable. More detailed dynamical studies are planned, and a normal mode analysis of the vibrational modes of C3H+3 is reported as the first step in that direction