Experiments on dissociative recombination on ArH+ show a small but nonzero cross section at low collision energies. Therefore, dissociative recombination of ArH+, as well as NeH+, is studied theoretically. Accurate ab initio fixed-nuclei electron-scattering calculations are performed to produce S-matrices that are used as input into a multichannel quantum defect treatment of the dissociation process. Results are compared to available experimental data.

We have studied low-energy mutual neutralization in reactions of Si-with Na+ and with K+ ions at the merged-beam double ion-beam storage-ring facility DESIREE. We made measurements with and without a significant fraction of the Si-ions in the metastable 2D level. The latter was achieved by state-selective photodetachment. The data reveal contributions from spin-forbidden mutual-neutralization reactions, i.e., violations of Wigner's spin-conservation rule. For the Na+ + Si-case, the dominating such reaction channel is obscured when metastable Si-ions are present in the beam.

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.

Radiation pressure from Lyman-alpha (Ly alpha scattering is a potentially dominant form of early stellar feedback, capable of injecting up to similar to 100 x more momentum into the interstellar medium (ISM) than ultraviolet continuum radiation pressure and stellar winds. Ly alpha feedback is particularly strong in dust-poor environments and is thus especially important during the formation of the first stars and galaxies. As upcoming galaxy formation simulations incorporate Ly alpha feedback, it is crucial to consider processes that can limit it to avoid placing Lambda-cold dark matter in apparent tension with recent JWST observations indicating efficient star formation at Cosmic Dawn. We study Ly alpha feedback using a novel analytical Ly alpha radiative transfer solution that includes the effects of continuum absorption, gas velocity gradients, Ly alpha destruction (e.g. by 2p -> 2s transitions), ISM turbulence, and atomic recoil. We verify our solution for uniform clouds using extensive Monte Carlo radiative transfer (MCRT) tests, and resolve a previous discrepancy between analytical and MCRT predictions. We then study the sensitivity of Ly alpha feedback to the aforementioned effects. While these can dampen Ly alpha feedback by a factor less than or similar to fewx 10, we find it remains greater than or similar to 5 - 100x stronger than direct radiation pressure and therefore cannot be neglected. We provide an accurate fit for the Ly alpha force multiplier M-F, suitable for implementation in subgrid models for galaxy formation simulations. Our findings highlight the critical role of Ly alpha feedback in regulating star formation at Cosmic Dawn, and underscore the necessity of incorporating it into simulations to accurately model early galaxy evolution.

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.

Context. Mutual neutralization (MN) between cations and anions plays an important role in determining the charge balance in certain astrophysical environments. However, empirical data for such reactions involving complex molecular species have been lacking due to challenges in performing experimental studies, leaving the astronomical community to rely on decades-old models with large uncertainties for describing these processes in the interstellar medium. Aims. Our aim is to investigate the MN reaction C-60(+) + C-60(-) -> C-60(* )+ C-60 for collisions at interstellar-like conditions. Methods. We studied the MN reaction between C-60(+) and C(60)(- )at collision energies of 100 meV using the Double ElectroStatic Ion Ring ExpEriment (DESIREE) and its merged beam capabilities. To aid in the interpretation of the experimental results, semiclassical modeling based on the Landau-Zener approach was performed for the studied reaction. Results. We experimentally identified a narrow range of kinetic energies for the neutral reaction products. Modeling was used to calculate the quantum state-selective reaction probabilities, absolute cross sections, and rate coefficients of these MN reactions, using the experimental results as a benchmark. We compared the MN cross sections with model results for electron attachment to C-60 and electron recombination with C-60(+). Conclusions. Our results show that it is crucial to take mutual polarization effects, the finite sizes, and the final quantum states of both molecular ions into account in order to obtain reliable predictions of MN rates expected to strongly influence the charge balance and chemistry in environments such as dense molecular clouds.

The problem of asymptotic non-adiabatic couplings in heavy particle collisions is treated using the reprojection method. The mixing matrix that mixes the asymptotic solutions of the coupled states to obtain appropriate boundary conditions is here derived to second order, yielding a faster convergence of the cross section. In addition, the reprojection method is implemented in a diabatic representation and applied to inelastic scattering of Li + Na and H + H collisions and to mutual neutralization in H+ + H- collisions.

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 have studied the stability of the smallest long-lived all carbon molecular dianion (C2-7 ) in new time domains and with a single ion at a time using a cryogenic electrostatic ion-beam storage ring. We observe spontaneous electron emission from internally excited dianions on millisecond timescales and monitor the survival of single colder C2- 7 molecules on much longer timescales. We find that their intrinsic lifetime exceeds several minutes-6 orders of magnitude longer than established from earlier experiments on C2- 7 . This is consistent with our calculations of vertical electron detachment energies predicting one inherently stable isomer and one isomer which is stable or effectively stable behind a large Coulomb barrier for C2- 7 -> C-7 thorn e- separation.

Sodium iodide (NaI) has, over the years, served as a prototype system in studies of non-adiabatic dynamics. Here, the charge transfer collision reactions Na+ + I- reversible arrow Na + I (mutual neutralization and ion-pair formation) are studied using an ab initio approach and the total and differential cross sections are calculated for the reactions. This involves electronic structure calculations on NaI to obtain adiabatic potential energy curves, non-adiabatic and spin-orbit couplings, followed by nuclear dynamics, treated fully quantum mechanically in a strictly diabatic representation. A single avoided crossing at 13.22 a(0) dominates the reactions, and the total cross sections are well captured by the semi-classical Landau-Zener model. Compared to the measured ion-pair formation cross section, the calculated cross section is about a factor of two smaller, and the overall shape of the calculated differential cross section is in reasonable agreement with the measured ion-pair formation differential cross section. Treating the Landau-Zener coupling as an empirical parameter of 0.05 eV, the measured total and differential cross sections are well captured when performing fully quantum mechanical cross section calculations including rotational coupling. A semi-empirical spin-orbit coupling model is also investigated, giving satisfactory estimation of the effects of spin-orbit interactions for the reactions.

We have studied the stability of the smallest long-lived all carbon molecular dianion (C_{7}^{2-}) in new time domains and with a single ion at a time using a cryogenic electrostatic ion-beam storage ring. We observe spontaneous electron emission from internally excited dianions on millisecond timescales and monitor the survival of single colder C_{7}^{2-} molecules on much longer timescales. We find that their intrinsic lifetime exceeds several minutes-6 orders of magnitude longer than established from earlier experiments on C_{7}^{2-}. This is consistent with our calculations of vertical electron detachment energies predicting one inherently stable isomer and one isomer which is stable or effectively stable behind a large Coulomb barrier for C_{7}^{2-}→C_{7}^{-}+e^{-} separation.

We measured the spontaneous decays of internally hot copper and silver dimer anions, Cu-2 and Ag-2, stored in one of the two ion-beam storage rings of the Double Electrostatic Ion Ring Experiment (DESIREE) at Stockholm University. A coincidence detection technique was utilized enabling essentially background-free measurements of Cu-2 -> Cu + Cu- and Ag-2 -> Ag + Ag- fragmentation rates. Furthermore, the total rates of neutral decay products (monomers and dimers) were measured and the relative contributions of fragmentation and electron emission (Cu-2 -> Cu2 + e- and Ag-2 -> Ag2 + e-) were deduced as functions of storage time. Fragmentation is completely dominant at early times. However, after about 20 ms of storage, electron emission is observed and becomes the leading decay path after 100 ms for both dimer anions. The branching ratios between fragmentation and electron emission (vibrationally assisted autodetachment processes) are very nearly the same for Cu-2 and Ag-2 throughout the present storage cycle of 10 seconds. This is surprising considering the difference between the electron affinities of the neutral dimers, Cu2 and Ag2, and the difference between the Cu-2 and the Ag- 2 dissociation energies.

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.

We study the electronic resonant states of H3 with energies above the potential energy surface of the H3+ ground state. These resonant states are important for the dissociative recombination of H3+ at higher collision energies, and previous studies have indicated that these resonant states exhibit a triple intersection. We introduce a complex generalization of the pseudo-Jahn-Teller model to describe these resonant states. The potential energies and the autoionization widths of the resonant states are computed with electron scattering calculations using the complex Kohn variational method, and the complex model parameters are extracted by a least-square fit to the results. This treatment results in a non-Hermitian pseudo-Jahn-Teller Hamiltonian describing the system. The non-adiabatic coupling and geometric phase are further calculated and used to characterize the enriched topology of the complex adiabatic potential energy surfaces.

An overview of recent experimental results of studies of negative atomic and molecular ions in the Double ElectroStatic Ion-Ring ExpEriment, DESIREE is given. Metastable level lifetimes in atomic negative ions have been measured by time-dependent laser photodetachment. Rotational relaxation of diatomic anions is studied by near-threshold photodetachment. Spontaneous decays of small metal cluster anions by electron emission and fragmentation is studied with decay-channel specificity. Finally, mutual neutralisation of pairs of negative and positive ions has been investigated with initial and final state selectivity.