We present a nonperturbative theory to describe multielectronic processes occurring in the course of collisions between an ion and a molecule. The approach is based on the expansion of the electronic scattering wave function onto asymptotic mono- or multicenter states with proper translational conditions and includes both static and dynamical electronic correlations. Therefore, it has a wide application range around intermediate impact velocities v$\approx${}ve, where ve is the averaged electron velocity in the initial state. As a first application, we report results on single- and double-electron capture processes in He2+-H2 collisions for impact energies ranging from 0.01 to 25~keV/u. Special emphasis on the prediction of cross sections for double-electron capture into doubly excited states of helium is addressed.

The injection of energetic neutral particles into the plasma of magnetic confinement fusion reactors is a widely-accepted method for heating such plasmas; various types of neutral beam are also used for diagnostic purposes. Accurate atomic data are required to properly model beam penetration into the plasma and to interpret photoemission spectra from both the beam particles themselves (e.g. beam emission spectroscopy) and from plasma impurities with which they interact (e.g. charge exchange recombination spectroscopy). This paper reviews and compares theoretical methods for calculating ionization, excitation and charge exchange cross sections applied to several important processes relevant to neutral hydrogen beams, including H + Be⁴⁺ and H + H⁺. In particular, a new cross section for the proton-impact ionization of H (1s) is recommended which is significantly larger than that previously accepted at fusion-relevant energies. Coefficients for an empirical fit function to this cross section and to that of the first excited states of H are provided and uncertainties estimated. The propagation of uncertainties in this cross section in modeling codes under JET-like conditions has been studied and the newly-recommended values determined to have a significant effect on the predicted beam attenuation. In addition to accurate calculations of collisional atomic data, the use of these data in codes modeling beam penetration and photoemission for fusion-relevant plasma density and temperature profiles is discussed. In particular, the discrepancies in the modeling of impurities are reported. The present paper originates from a Coordinated Research Project (CRP) on the topic of fundamental atomic data for neutral beam modeling that the International Atomic Energy Agency (IAEA) ran from 2017 to 2022; this project brought together ten research groups in the fields of fusion plasma modeling and collisional cross section calculations. Data calculated during the CRP is summarized in an appendix and is available online in the IAEA’s atomic database, CollisionDB.

Interatomic Coulombic electron capture (ICEC) is an environment-assisted process in which a free electron can efficiently attach to an ion, atom or molecule by transferring the excess energy to a neighboring species. Absolute cross sections are necessary to evaluate the relative importance of this process. In this work, we employ the $R$-matrix method to compute ab initio these cross sections for a singly charged neon ion embedded in small helium clusters. Our results show that the ICEC cross sections are several orders of magnitude higher than anticipated and dominate other competing processes. Electron energy loss spectra on an absolute scale are provided for the ${\mathrm{Ne}}^{+}$@${\mathrm{He}}_{20}$ cluster. Such spectra exhibit an unambiguous signature of the ICEC process. The finding is expected to stimulate experimental observations.

Potential energy surfaces (PESs) play a central role in our understanding of chemical reactions. Despite the impressive development of efficient electronic structure methods and codes, such computations still remain a difficult task for the majority of relevant systems. In this context, artificial neural networks (NNs) are promising candidates to construct the PES for a wide range of systems. However, the choice of suitable molecular descriptors remains a bottleneck for these algorithms. In this work, we show that a principal component analysis (PCA) is a powerful tool to prepare an optimal set of descriptors and to build an efficient NN: this protocol leads to a substantial improvement of the NNs in learning and predicting a PES. Furthermore, the PCA provides a means to reduce the size of the input space (i.e., number of descriptors) without losing accuracy. As an example, we applied this novel approach to the computation of the high-dimensional PES describing the keto–enol tautomerism reaction occurring in the acetone molecule.

We study photon- and ion-induced electron emission from one-electron diatomic molecular ions. For photon impact we find an oscillatory behavior of the asymmetry parameter $\beta$ and of the total cross section which can be related to the presence of Cooper minima in the partial cross sections. This behavior can also be related to coherent electron emission from the two centers of the molecule. By comparing the results for homonuclear and heteronuclear molecular ions, we show that the periodic structures predicted by both pictures appear clearly only for the former targets. This conclusion is supported by study of the electron emission spectra induced by swift ion impact.

Almost 10 years ago, energetic neutral hydrogen atoms were detected after a strong-field double ionization of ${\mathrm{H}}_2$. This process, called “frustrated tunneling ionization,” occurs when an ionized electron is recaptured after being driven back to its parent ion by the electric field of a femtosecond laser. In the present study we demonstrate that a related process naturally occurs in clusters without the need of an external field: we observe a charge hopping that occurs during a Coulomb explosion of a small helium cluster, which leads to an energetic neutral helium atom. This claim is supported by theoretical evidence. As an analog to frustrated tunneling ionization, we term this process “frustrated Coulomb explosion.”