Ab initio study of low-energy electron-methane scattering

Joint experimental and theoretical study on electron scattering from titanium tetrachloride (TiCl4) molecule

Absolute grand-total cross section for electron scattering from titanium tetrachloride, TiCl₄, molecule was measured at electron-impact energies ranging from 0.3 to 300 eV, in the linear electron-transmission experiment. The elastic integral, differential, momentum transfer, and total ionization cross sections for TiCl₄ molecule were also calculated for low and intermediate collisional energies at the level of various theories. The low-energy elastic integral, differential, and momentum transfer cross sections were calculated with the Schwinger multichannel method implemented with pseudopotentials, in the static-exchange and static-exchange plus polarization levels of approximation, for energies up to 30 eV. The integral cross section calculated for low-energy electron scattering with the R-matrix method within the static-exchange and static-exchange plus polarization approximations for energies up to 15 eV are also reported. By the inspection of the cross sections, the presence of resonances is discussed. In particular, the calculated integral cross sections and the measured total cross section display a minimum at around 1 eV, which is consistent with the presence of a Ramsauer-Townsend minimum and a sharp increase at low energies, which is consistent with the presence of a virtual state. Additionally, interactions in elastic and ionization channels for intermediate collision energies were investigated with the additivity rule and the binary-encounter-Bethe methods.

Low Energy Positron Scattering by F and F2

We have determined integral and differential elastic cross sections for positron scattering with atomic F and F₂ molecules from 0.01 to 1000 eV with different theoretical approaches. Higher order polarizabilities were successively considered in the scattering potential, and we observed that the differential and integral cross sections converge only when the pure dipole second hyperpolarization level is taken into account, demonstrating the importance of the hyperpolarization effects on the low energy positron scattering dynamics. Ramsauer structures were found for both targets, and in the particular case of the F atom, the effect of its quadrupole moment proved to be relevant around the position of the Ramsauer minimum, which was determined together with the scattering length for each of the adopted polarization schemes. A relation of these quantities with the matching radius between the correlation and polarization interactions is also discussed. Agreement between the model potential and ab initio cross sections for the lowest level of target polarization is found for F₂. Since there is no previous theoretical or experimental determination of cross sections for these targets, we compared the differential cross sections of F₂ with experimental data available for N₂ and O₂ and found reasonable agreement.

BEAMDB and MOLD—Databases at the Serbian Virtual Observatory for Collisional and Radiative Processes

In this contribution we present a progress report on two atomic and molecular databases, BEAMDB and MolD, which are web services at the Serbian virtual observatory (SerVO) and nodes within the Virtual Atomic and Molecular Data Center (VAMDC). The Belgrade Electron/Atom (Molecule) DataBase (BEAMDB) provides collisional data for electron interactions with atoms and molecules. The Photodissociation (MolD) database contains photo-dissociation cross sections for individual rovibrational states of diatomic molecular ions and rate coefficients for the chemi-ionisation/recombination processes. We also present a progress report on the major upgrade of these databases and plans for the future. As an example of how the data from the BEAMDB may be used, a review of electron scattering from methane is described.

Temporary Anion States of Ethene Interacting with Single Molecules of Methane, Ethane, and Water

When an excess electron is added into the π* orbital of ethene, the resulting anion decays by electron autodetachment; that is, it represents an electronic state referred to as a temporary anion or resonance state. Here, the influence of a cluster environment on the energy and lifetime of this state is investigated. The clusters considered are ethene···CH₄, ethene···C₂H₆, and ethene···H₂O. Most of these clusters are systematically constructed so that the solvent interacts with the π system in a specific way, and are thus by construction not minima with respect to all intermolecular degrees of freedom. However, for water, in addition, a minimal energy structure is examined. Systematic variation of the solvent and solvation geometry allows us to identify trends regarding effects due to polarizability, excluded volume, and polarity of the solvent molecules. The resonance parameters of ethene and all temporary cluster anions are computed with the symmetry-adapted cluster-configuration interaction electronic structure method in combination with a complex absorbing potential. This method is well-established for small to intermediate sized molecules. In addition to the study of the solvation effects themselves, the question of how many basis functions are needed on the closed-shell solvating unit is examined.

Signatures of bond formation and bond scission dynamics in dissociative electron attachment to methane

We show that a single T₂ resonance can explain the 10 eV dissociative electron attachment peak in methane.

Low-energy electron collisions with gas-phase uracil

We have studied gas-phase collisions between slow electrons and uracil molecules with a view to understanding the resonance structure of the scattering cross section. Our symmetry-resolved results for elastic scattering, computed in the fixed-nuclei, static-exchange and static-exchange-plus-polarization approximations, provide locations for the expected pi* shape resonances and indicate the possible presence of a low-energy sigma* resonance as well. Electron-impact excitation calculations were carried out for low-lying triplet and singlet excitation channels and yield a very large singlet cross section. We discuss the connection between the resonances found in our elastic cross section and features observed in dissociative attachment.