Jimena D. Gorfinkiel

GTOBAS: fitting continuum functions with Gaussian-type orbitals☆

GTOBAS is a program for fitting Gaussian-type orbitals (GTOs) to Bessel and Coulomb functions over a finite range. The exponents of the GTOs are optimized using the method of Nestmann and Peyerimhoff [J. Phys. B 23 (1990) L773]. The appended module NUMCBAS provides the numerical Bessel and Coulomb functions required as input for the program. The use of GTO continuum basis sets is particularly important in electron–molecule scattering calculations when polyatomic targets are involved. Sample results for such calculations are also discussed.

Low-energy electron collisions with tetrahydrofuran

We have performed calculations for electron collisions with tetrahydrofuran (THF) using the UK molecular R-matrix codes. This is the largest molecule ever treated with the R-matrix method, and the only biologically relevant molecule of this size studied theoretically in the inelastic regime. We report ab initio integral cross section for incident energies up to 10 eV. No shape resonances have been found for this system, but a few core-excited resonances are present.

Elastic and inelastic cross sections for low-energy electron collisions with pyrimidine.

We present theoretical elastic and electronic excitation cross sections and experimental electronic excitation cross sections for electron collisions with pyrimidine. We use the R-matrix method to determine elastic integral and differential cross sections and integral inelastic cross sections for energies up to 15 eV. The experimental inelastic cross sections have been determined in the 15-50 eV impact energy range. Typically, there is quite reasonable agreement between the theoretical and experimental integral inelastic cross sections. Calculated elastic cross sections agree very well with prior results.

Low-energy electron collisions with water: elastic and rotationally inelastic scattering

Differential, integral and momentum transfer cross sections for the vibrationally elastic and rotationally inelastic scattering of electrons from water at low collision energies (E < 7 eV) are reported. The R-matrix method is used to compute the body-fixed T-matrices while the scattering calculations are performed within the fixed-nuclei approximation corrected with the standard Born-closure formula. Our calculations are compared with the very recent experimental results of Cho et al (2003 Radiat. Phys. Chem. 68 115). The differential and momentum transfer cross sections are in good agreement with the experimental results. The relative contribution of the rotationally inelastic processes is investigated in some detail. In particular, the importance of the pure elastic process at very low energy is emphasized.

Electron impact dissociative excitation of water within the adiabatic nuclei approximation

The R-matrix method is used to calculate dissociative excitation cross sections for the four lowest-lying electronically excited states of H2O in the energy range 5-15 eV. For the first time calculations are performed taking into account the nuclear motion by means of an adaptation of the adiabatic nuclei approximation. Cross sections are compared with previous and new fixed-nuclei results and also experiments. Resonance positions and widths are calculated for different geometries of the water molecule.

Electron attachment to molecules in a cluster environment

Low-energy dissociative electron attachment (DEA) to the CF(2)Cl(2) and CF(3)Cl molecules in a water cluster environment is investigated theoretically. Calculations are performed for the water trimer and water hexamer. It is shown that the DEA cross section is strongly enhanced when the attaching molecule is embedded in a water cluster, and that this cross section grows as the number of water molecules in the cluster increases. This growth is explained by a trapping effect that is due to multiple scattering by water molecules while the electron is trapped in the cluster environment. The trapping increases the resonance lifetime and the negative ion survival probability. This confirms qualitatively existing experiments on electron attachment to the CF(2)Cl(2) molecule placed on the surface of H(2)O ice. The DEA cross sections are shown to be very sensitive to the position of the attaching molecule within the cluster and the orientation of the electron beam relative to the cluster.

Electron–H+3 collisions at intermediate energies

A new procedure is presented for the ab initio study of electron?molecule collision at energies straddling the target ionization threshold. The R-matrix with pseudostates method, which allows for the inclusion of discretized continuum states in a close-coupling expansion, is adapted to molecular targets using even-tempered basis sets. Calculations for electron collisions with the H+3 molecular ion provide converged polarizabilities, electronic excitation and ionization cross sections.

Electron impact ionization of small molecules at intermediate energies: the molecular R-matrix with pseudostates method

A procedure for the ab initio study of electron-molecule collisions at intermediate energies is presented in detail. The molecular R-matrix with pseudostates method is based on the inclusion of discretized continuum states in the close-coupling expansion. This method allows, for the first time, the calculation of totally ab initio cross sections for electron impact ionization of molecules as well as for electronic excitation above the first ionization threshold. The method is general and can be applied to multielectron targets. Results for collisions with H-3(+) and H-2 are presented. Numerical considerations necessary for performing a successful calculation are detailed.

Shape and core excited resonances in electron collisions with diazines

We present a comprehensive ab-initio study of electron collisions with pyrazine, pyrimidine, and pyridazine. The emphasis is placed on the identification and characterization of electron resonances in these systems. We use the R-matrix method and show that analysing the time-delay reveals resonances whose signature is not visible in the eigenphase sums. In addition to the well known π∗ resonances below 5 eV, we find three core-excited shape resonances in the energy range 5.5-8.5 eV and a few Feshbach resonances in the dipolar molecules. Additionally, 11 resonances with little effect on the elastic scattering from ground state diazines (but significant effect in elastic collisions with the molecules in an excited state) are found and characterized. We correlate these resonances across the three molecules and discuss their possible correspondence to resonances described in earlier studies on uracil.

Elastic and inelastic low-energy electron collisions with pyrazine.

We present results of ab-initio scattering calculations for electron collisions with pyrazine using the R-matrix method, carried out at various levels of approximation. We confirm the existing experimental and theoretical understanding of the three well-known π∗ shape resonances. In addition, we find numerous core-excited resonances (above 4.8 eV) and identify their most likely parent states. We also present differential cross sections, showing high sensitivity to the scattering model chosen at low energies. We make recommendations regarding the selection of models for scattering calculations with this type of targets.