Table of contents

In this work we present calculated cross sections for elastic scattering of low-energy positrons by molecules, obtained with the Schwinger multichannel method. We extended a previous study on the role of polarization effects in the positron-allene scattering and also present some preliminary results for positron collisions with cyclopentane. We also compare the differential cross sections of benzene and its azaderivative pyrimidine at selected energies, where similar angular behaviour was found.

We present negative ion formation from collisions of neutral potassium atoms with neutral phenylboronic acid C₆H₅B(OH)₂ molecules in the lab frame energy range from 10 to 1000 eV. From the assignment of the negative ion time-of-flight (ToF) mass spectra, BO⁻ is the main fragment detected at energies below 100 eV, however above 100 eV BO₂⁻ is the predominant fragment accounting on average for 30% of the total anion yield for collision energies above 250 eV. The rich fragmentation pattern results in the formation of more than thirty fragment anions, with twenty boron containing ions.

Resonances play an important role in a number of atomic and molecular processes. Identifying and characterising resonances in electron scattering is essential as they can both enhance a number of processes (e.g. electronic and vibrational excitation) and are crucial in others like dissociative electron attachment and dissociative recombination. We discuss recent theoretical studies of shape and core-excited resonances, both in isolated molecules of biological relevance and in small molecular clusters. The latter are investigated to understand the effect of the environment, in particular hydration, in electron collisions in biological matter.

We report recent results of mass-resolved anion fragment momentum imaging experiments to investigate dissociative electron attachment to formic acid, for incident energies between 5 eV and 9 eV. A remarkable site-selectivity is found for a resonance at 8.5 eV by comparing anion fragment yields for two deuterated isotopologues of formic acid. This results in an H⁻ fragment from the O-H bond of the transient anion, with negligible contribution from C-H break. In contrast, a lower-energy resonance at 7.1 eV dissociates by C–H or O–H break to produce H⁻ and the neutral radicals HOCO or HCOO.

We present experimental results for dissociative electron attachment to sulfur dioxide near the 7.5 eV resonance. The dissociation channels leading to O⁻, S⁻ and SO⁻ ions formation have been investigated with a velocity map imaging technique. While the O⁻ and SO⁻ ions formed by dissociative electron attachment show angular distributions characteristic to a combination of A₁+B₂ symmetry, the S⁻ ions show angular distributions resulting from changes in the orientation of the dissociating bond due to bending mode vibrations implying the failure of axial-recoil approximation.

Single electron attachment to a molecule may invoke quantum coherence in different angular momentum transfer channels. This has been observed in the 14 eV dissociative electron attachment resonance in molecular hydrogen where a coherent superposition of two negative ion resonant states of opposite parity is created, with the s and p partial waves of the electron contributing to the attachment process. Interference between the two partial wave contributions leads to a forward – backward asymmetry in the angular distribution of the product negative ions. Since these two resonant states dissociate to the same n = 2 state of H and H⁻, this asymmetry is further modified due to interference between the two paths of the dissociating molecular negative ion along different potential energy curves. This interference manifests as a function of the electron energy as well as isotopic composition. This case is akin to the quantum interference observed in photodissociation by one-photon vs two-photon absorption.

Molecular dications are important metastable species present in a variety of environments including, but not restricted to, planetary atmospheres and the human body when submitted to radiation. On the other hand, symmetric molecular dications cannot be separated from the cation corresponding to the molecule broken in half (e.g. ${{\rm{N}}}_{2}^{++}$ and N⁺) in stantard mass spectrometers. The DETOF technique allows one to identify the symmetric molecular dication, obtaining absolute cross sections related to their production. Results for dications of molecular nitrogen, molecular oxygen, ethylene and benzene are discussed here, giving emphasis on the mechanisms behind their production. It is seen that, while most metastable molecular dications result from a combination of direct double ionization and Auger-like processes, ${{\rm{N}}}_{2}^{++}$ and ${{\rm{O}}}_{2}^{++}$ represent so-called pure cases, where the dication is produced only by, respectively, double direct ionization or post-collisional Auger-like decay.

To investigate how replacement of H atom with methyl group (CH₃) – in tetrahedral compounds of carbon, silicon and germanium – affects electron scattering process, total cross sections (TCS) for electron scattering from C(CH₃)₄, Si(CH₃)₄ and Ge(CH₃)₄ molecules have been compared with data for CH₄, SiH₄ and GeH₄ molecules. All examined data have been obtained with the same experimental setup. The shape of all discussed TCS energy dependences is very similar and is characterized by a dominant maximum peaked below 10 eV. For methylated compounds a gentle structure is also visible on high energy slope of main enhancement, between 10 – 20 eV. A simple formula for TCS evaluation for partially methylated carbon, silicon and germanium compounds is also proposed.

The recently developed many-body-theory approach for studying low-energy positronium-atom interactions is applied to calculate positronium scattering by atomic hydrogen. Calculations are carried out for the case where the two electrons are in a spin-triplet state. The elastic scattering phase shifts and cross sections are compared with existing coupled-channel, stochastic-variational, and Kohn-variational results, and excellent agreement is obtained.

Dissociative electron attachment (DEA) to molecule plays a key role in atmosphere, interstellar space and ionization damages of biological tissue. Experimental DEA studies of polyatomic molecules in gas phase provide the dynamics details that are the fundamentals to establish the physicochemical models of the electron-induced reactions in complicated environments. Since 2012, we successively set up two ion-velocity-map-imaging apparatuses, and accomplished a series of experimental studies of the DEA dynamics. Here is a brief review about our progresses on polyatomic molecules.

We describe recent theoretical efforts to explain the observed similarity between electron and positronium (Ps) scattering with both atoms and molecules. In order to perform these calculations at a more ab initio level we have developed a Free Electron Gas (FEG) approximation to the exchange and correlation potentials similar to that used in electron-molecule scattering. The FEG results give good agreement with experiment above the Ps ionization threshold and exhibit resonance features below the threshold. To take account of the effect of orthogonality between the scattered electron and the target wavefunctions we have used an orthogonalizing pseudopotential (OPP). The FEG plus OPP results give good agreement with experiment for Ps-rare-gas scattering. Preliminary results for Ps-H₂ scattering also agree well with experiment and demonstrate the importance of including orthogonality for scattering below the Ps ionization threshold.

We present a 4-body calculation of scattering between an antihydrogen atom ($\bar{{\rm{H}}}$) and a positronium (Ps) aiming at the prediction of cross sections for the production of antihydrogen positive ions ${\bar{{\rm{H}}}}^{+}$. The antihydrogen positive ions are expected to be a useful source of ultra-cold anti-atoms for the test of matter-antimatter gravity. We convert the Schrödinger equation to a set of coupled integro-differential equations that involve intermediate states which facilitate the internal region description of the scattering wavefunction. They are solved using a compact finite difference method. Our framework is extended to scattering between an excited Ps and H. Cross sections of the reactions, Ps (1s/2s/3s) $\bar{{\rm{H}}}\to {{\rm{e}}}^{-}+{\bar{{\rm{H}}}}^{+}$, in s-wave collisions, are calculated. It is found that the reactions originating from Ps (1s/2s) + $\bar{{\rm{H}}}$ produce ${\bar{{\rm{H}}}}^{+}$ with a constant cross section within 0.05 eV above the threshold while the reaction cross section from Ps (3s) decreases as the collision energy increases in the same energy interval. Just above the threshold, the cross section of ${\bar{{\rm{H}}}}^{+}$ production from Ps (3s) + $\bar{{\rm{H}}}$ in s-wave collision is 7.8 times larger than that from Ps (1s) + $\bar{{\rm{H}}}$ in s-wave and 2.3 times larger than that from Ps (2s) + $\bar{{\rm{H}}}$ in s-wave. The near-threshold de-excitation reaction from Ps (3s) + $\bar{{\rm{H}}}$ occurs more rapidly than the ${\bar{{\rm{H}}}}^{+}$ production.

The present status of the fully-relativistic nonperturbative calculations of the fundamental atomic processes with twisted electrons is presented. In particular, the elastic (Mott) scattering, the radiative recombination, and for the very first time, the Bremsstrahlung processes are considered. The electron-ion interaction is accounted for in a nonperturbative manner, that allows obtaining reliable results for heavy systems. We investigate the influence of the "twistedness" of the incoming electron on the angular and polarization properties of the emitted electrons and photons for the elastic and inelastic scattering, respectively. It is found that these properties exhibit a strong dependence on the opening angle of the vortex electron beam in all processes considered.