Dissociative electron attachment to NaCN is investigated theoretically by combining electron scattering calculations, structure calculations and wave packet dynamics. Non-adiabatic couplings between resonant states and electronically bound states of NaCN ^- are considered. The calculated cross section has a threshold of 0.68 eV. Due to the very narrow autoionization widths of the electronic resonant states, the magnitude of the cross section is very low. Hence, dissociative electron attachment is not a pathway for forming CN ^- in interstellar space.

Motivated by the need to model the plasma at ITER, the cross section - both total and differential - and branching ratios for mutual neutralization in collisions of B^+ with H^- are calculated using a close coupling approach. Potential energy curves and non-adiabatic coupling elements of seven electronic states of BH in ^1Σ ^+ symmetry are computed using the multireference configuration interaction method.

We have experimentally studied dissociative recombination (DR) of electronically and vibrationally relaxed ArH+ in its lowest rotational levels, using an electron-ion merged-beams setup at the Cryogenic Storage Ring. We report measurements for the merged-beams rate coefficient of ArH+ and compare it to published experimental and theoretical results. In addition, by measuring the kinetic energy released to the DR fragments, we have determined the internal state of the DR products after dissociation. At low collision energies, we find that the atomic products are in their respective ground states, which are only accessible via nonadiabatic couplings to neutral Rydberg states. Published theoretical results for ArH+ have not included this DR pathway. From our measurements, we have also derived a kinetic temperature rate coefficient for use in astrochemical models. Published by the American Physical Society 2024

The mutual neutralization reaction in collisions of Li+ with CN- is a promising candidate for rigorous multi-dimensional ab initio studies of atom-molecule charge transfer processes. The reaction is driven by the non-adiabatic interaction between the lowest two (1)A ' electronic states at large Li-CN distances, resulting in a large cross section for mutual neutralization. As a first step, the relevant adiabatic potential energy surfaces and non-adiabatic interaction are computed ab initio, and the process is studied quantum mechanically using the vibrational sudden approximation, where the vibrational and rotational motions of the CN molecule are assumed to be frozen during the collision.

We have measured the photodissociation of few-keV OD+ molecular ions into either D+ + O or O+ + D final products. The three-dimensional momentum imaging measurements of the light and massive fragments in coincidence were enabled by using an upgraded two-detector setup. In this work, we show that absorption of a single 790 or 395 nm photon excites the OD+ from its electronic ground state to the B (3)Sigma(-)state, which dissociates to the O+(S-4) + D dissociation limit. To reach the other nearly degenerate dissociation limit, D+ + O(P-3), a unimolecular charge transfer, B (3)Sigma(-) to B (3)Sigma(-), transition is required following the same photoexcitation. The measured branching ratio of these dissociation channels is a direct measure of the charge transfer transition probability. This measured probability as a function of energy above the dissociation limit agrees well with our calculations.

We applied reaction microscopy to elucidate fast non-adiabatic dissociation dynamics of deuterated water molecules after direct photo-double ionization at 61 eV with synchrotron radiation. For the very rare D+ + O+ + D breakup channel, the particle momenta, angular, and energy distributions of electrons and ions, measured in coincidence, reveal distinct electronic dication states and their dissociation pathways via spin–orbit coupling and charge transfer at crossings and seams on the potential energy surfaces. Notably, we could distinguish between direct and fast sequential dissociation scenarios. For the latter case, our measurements reveal the geometry and orientation of the deuterated water molecule with respect to the polarization vector that leads to this rare 3-body molecular breakup channel. Aided by multi-reference configuration-interaction calculations, the dissociation dynamics could be traced on the relevant potential energy surfaces and particularly their crossings and seams. This approach also unraveled the ultrafast time scales governing these processes.

We have developed a method for which a variety of reactive scattering processes involving the H2 reaction complex can be studied using the same set of potential curves and couplings. The method is based on a close-coupling approach in a strict diabatic representation. By rigorously incorporating nonadiabatic couplings among bound states, we enable the computation of final-state distributions. Loss into the ionization continuum is accounted for with a nonlocal complex potential matrix. The method has successfully been applied in studies of H++H− mutual neutralization and H(1s)+H(ns) associative ionization. In this paper, we investigate the applicability of this method to dissociative recombination and resonant ion-pair formation in electron collisions with HD+. The importance of a nonlocal description of autoionization is demonstrated. Calculated cross sections and final-state distributions are compared with results from experiments and previous theoretical studies. Published by the American Physical Society 2024

The total and differential cross-sections and final state distribution for mutual neutralization in collisions of Li+ with O- were calculated using an ab initio quantum mechanical approach based on potential energy curves and non-adiabatic coupling elements computed with the multi-reference configuration interaction method. The final state distributions favored channels with excited oxygen states, indicating a strong effect of electron correlation, and the electron transfer could not be described by a simple one-electron exchange process.

Associative ionization in collisions of H+ + H- as well as H(1s) + H(ns) with n = 2, 3, 4 is studied theoretically. Relevant adiabatic potential curves and nonadiabatic couplings are calculated ab initio and the autoionization from the lowest electronic resonant states in the 11+g/u and 31+g/u symmetries are considered. The cross sections are obtained by solving the coupled Schrodinger equation, including a complex potential matrix, in a strict diabatic representation. The importance of using a nonlocal description of autoionization is investigated. Associative ionization is also studied for different isotopes of hydrogen. Calculated cross sections are compared with results from measurements.

We present an investigation of the relaxation dynamics of deuterated water molecules after direct photo-double ionization at 61 eV. We focus on the very rare D+ + O+ + D reaction channel in which the sequential fragmentation mechanisms were found to dominate the dynamics. Aided by theory, the state-selective formation and breakup of the transient OD+(a1Δ, b1Σ+) is traced, and the most likely dissociation path-OD+: a1Δ or b1Σ+ → A 3Π → X 3Σ- → B 3Σ--involving a combination of spin-orbit and non-adiabatic charge transfer transitions is determined. The multi-step transition probability of this complex transition sequence in the intermediate fragment ion is directly evaluated as a function of the energy of the transient OD+ above its lowest dissociation limit from the measured ratio of the D+ + O+ + D and competing D+ + D+ + O sequential fragmentation channels, which are measured simultaneously. Our coupled-channel time-dependent dynamics calculations reproduce the general trends of these multi-state relative transition rates toward the three-body fragmentation channels.

We present the relaxation dynamics of deuterated water molecules via autoionization, initiated by the absorption of a 61 eV photon, producing the very rare D+ + O+ + D breakup channel. We employ the COLd target recoil ion momentum spectroscopy method to measure the 3D momenta of the ionic fragments and emitted electrons from the dissociating molecule in coincidence. We interpret the results using the potential energy surfaces extracted from multi-reference configuration interaction calculations. The measured particle energy distributions can be related to a super-excited monocationic state located above the double ionization threshold of D2O. The autoionized electron energy shows a sharp distribution centered around 0.5 eV, which is a signature of the atomic oxygen autoionization occurring in the direct and sequential dissociation processes of D2O+* at a large internuclear distance. In this way, an O+ radical fragment and a low-energy electron are created, both of which can trigger secondary reactions in their environment.

The total and differential cross sections of mutual neutralization in H+ + H- collisions are calculated ab initio and fully quantum mechanically for energies between 0.001 and 600 eV. Effects which have not previously been considered in studies on mutual neutralization (MN) for this system, such as inclusion of rotational couplings and autoionization, are investigated. Adiabatic potential curves corresponding to the relevant states of 1Eg+, 1Eu+,1 pi g and1 pi u symmetries as well as radial and rotational nonadiabatic couplings are computed ab initio. A quasidiabatic model is developed and applied in order to investigate the importance of higher excited states as well as the inclusion of autoionization. Molecular data for the lowest electronic resonant state in each symmetry are obtained by performing electron scattering calculations. It is shown that rotational couplings cause a significant increase of the total MN cross section while autoionization plays a minor role as a loss mechanism. Additionally, a differential cross section is obtained that is symmetric around 0 = 90 degrees. This result is in disagreement with a previous theoretical calculation where it was found that the differential cross section is dominated by backwards scattering.

Received 13 November 2020DOI:https://doi.org/10.1103/PhysRevA.102.069902©2020 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAtomic & molecular collisionsChemical reactionsElectron & positron scatteringScattering of atoms, molecules, clusters & ionsScattering theoryAtomic, Molecular & Optical

Synopsis The mutual neutralization reaction of Cl− + C+ → Cl + C is studied from first principles. The cross section and the thermal rate coefficient are calculated for low collision energies, and the effect of spin-orbit coupling is estimated using a semi-empirical method. Recent experiment has measured a thermal rate coefficient [1], which allows for a comparison between theoretical and experimental studies of the reaction.

Results from quantum mechanical ab initio studies on mutual neutralization in collision of H+ with H- and H-2(+) with H- are reported. The theoretical studies require computations of the potentials as well as the non-adiabatic interactions among highly excited electronic states. The electronic states are transformed to a diabatic representation and the coupled radial Schrodinger equation is solved using a logarithmic derivative method. For H-2(+) collisions with H-, the process is studied using reduced dimensionality. Calculated cross sections and final state distributions are compared with measurements.

Synopsis Dissociative electron attachment (DEA) to the molecule MgCN and its isomer MgNC have been proposed as possible sources of CN − in the interstellar media. We have carried out electron scattering calculations using the Complex Kohn Variational Method as a function of the internal degrees of freedom to obtain the resonance energy surfaces and autoionization widths. We use this data as input to form the Hamiltonian relevant to the nuclear dynamics. The multidimensional wave equation is solved using the MultiConfiguration Time-Dependent Hartree (MCTDH) approach. We compute the DEA cross sections.

The cross section and final state distribution for mutual neutralization in collisions of H+ with Cl- have been calculated using an ab initio quantum mechanical approach. It is based on potential energy curves and nonadiabatic coupling elements for the six lowest 1Σ+ states of HCl computed with the multireference configuration interaction method. The reaction is found to be driven by nonadiabatic interactions occurring at relatively small internuclear distances (R < 6 a0). Effects on the mutual neutralization cross section with respect to the asymptotic form of the potential energy curves, inclusion of closed channels, as well as isotopic substitution are investigated. The effect of spin-orbit interaction is investigated using a semiempirical model and found to be small. A simple two-state Landau-Zener calculation fails to predict the cross section.