Sergio d’A. Sanchez

Low-energy electron collisions with pyrrole

We report cross sections for low-energy elastic electron scattering by pyrrole, obtained with the Schwinger multichannel method implemented with pseudopotentials. Our calculations indicate pi( *) shape resonances in the B(1) and A(2) symmetries, and two sigma( *) resonances in the A(1) symmetry (the system belongs to the C(2v) point group). The present assignments of pi( *) resonances are very close to those previously reported for the isoelectronic furan molecule, in agreement with electron transmission spectra. The lowest-lying sigma( *) anion is localized on the N-H bond and provides a dissociation coordinate similar to those found in the hydroxyl groups of organic acids and alcohols. This sigma(NH) ( *) resonance overlaps the higher-lying pi( *) resonance (possibly both pi( *) states) and could give rise to direct and indirect dissociation pathways, which arise from electron attachment to sigma( *) and pi( *) orbitals, respectively. The photochemistry of pyrrole and 9-H adenine is similar, in particular with respect to the photostability mechanism that allows for the dissipation of the photon energy, and we believe pyrrole would also be a suitable prototype for studies of dissociative electron attachment (DEA) to DNA bases. We point out the connection between the mechanisms of photostability and DEA since both arise from the occupation of sigma( *) and pi( *) orbitals in neutral excited states and in anion states, respectively.

Low-energy positron scattering from iodomethane

We report experimental total cross sections (TCSs) and calculated elastic integral cross sections (ICSs) for positron collisions with iodomethane (methyl iodide, CH3I). The experimental TCSs were obtained with a linear transmission technique, for energies from 0.1 up to 50 eV. The present TCS data agree well with those previously reported (Kimura et al 2001 J. Chem. Phys. 115 7442) at higher energies (above 7 eV), but significant discrepancies are found at the lower energies. The present ICS computations were performed with the Schwinger multichannel method (SMC) and the Born dipole approximation in the incident energy range from 0.1 eV up to 10 eV. Iodomethane poses a great challenge to theoretical descriptions of the collisions dynamics. In addition to the neglect of inelastic channels, the main difficulty found in the SMC approach is related to numerical limitations that prevent a thorough description of correlation–polarization effects. Although our ICS calculations do not compare well with the present TCS data, the results are encouraging, as iodomethane would challenge all the presently available computational approaches.

Positron and electron scattering by glycine and alanine: Shape resonances and methylation effect

We report integral cross sections (ICSs) for both positron and electron scattering by glycine and alanine amino acids. These molecules differ only by a methyl group. We computed the scattering cross sections using the Schwinger multichannel method for both glycine and alanine in different levels of approximation for both projectiles. The alanine ICSs are greater in magnitude than the glycine ICSs for both positron and electron scattering, probably due to the larger size of the molecule. In electron scattering calculations, we found two resonances for each molecule. Glycine presents one at 1.8 eV, and another centered at around 8.5 eV, in the static-exchange plus polarization (SEP) approximation. The ICS for alanine shows one resonance at 2.5 eV and another at around 9.5 eV, also in SEP approximation. The results are in good agreement with most of the data present in the literature. The comparison of the electron scattering ICSs for both molecules indicates that the methylation of glycine destabilizes the resonances, shifting them to higher energies.

Positron collisions with C3H6 isomers

We report calculated elastic integral (ICS) and differential (DCS) cross sections for positron collisions with C3H6 isomers, propene and cyclopropane. The ICSs computations were performed with the Schwinger multichannel method, at the static plus polarization level of approximation from 0.001 up to 8 eV. These ICSs are compared with the only experimental total cross sections (TCSs) available in literature for low energy positron scattering by these isomers [Makochekanwa C et al 2008 Phys. Rev. A 77 042717]. Even though there are significant differences between the two molecules (for instance propene has a dipole moment while cyclopropane has not), the ICSs for both systems are remarkably similar. This is mainly due to the fact that the dipole moment of propene is very small (0.39 D), and the increase of cyclopropanes ICS at low impact energy is possibly due to the presence of a virtual state. This same resemblance occurs for the DCSs at lower energies, with the differences between the isomers becoming clearer as the energy increases.

Theoretical and experimental study on electron interactions with chlorobenzene: Shape resonances and differential cross sections

In this work, we report theoretical and experimental cross sections for elastic scattering of electrons by chlorobenzene (ClB). The theoretical integral and differential cross sections (DCSs) were obtained with the Schwinger multichannel method implemented with pseudopotentials (SMCPP) and the independent atom method with screening corrected additivity rule (IAM-SCAR). The calculations with the SMCPP method were done in the static-exchange (SE) approximation, for energies above 12 eV, and in the static-exchange plus polarization approximation, for energies up to 12 eV. The calculations with the IAM-SCAR method covered energies up to 500 eV. The experimental differential cross sections were obtained in the high resolution electron energy loss spectrometer VG-SEELS 400, in Lisbon, for electron energies from 8.0 eV to 50 eV and angular range from 7(∘) to 110(∘). From the present theoretical integral cross section (ICS) we discuss the low-energy shape-resonances present in chlorobenzene and compare our computed resonance spectra with available electron transmission spectroscopy data present in the literature. Since there is no other work in the literature reporting differential cross sections for this molecule, we compare our theoretical and experimental DCSs with experimental data available for the parent molecule benzene.

Near-threshold vibrational excitation of acetylene by positron impact

We report vibrational excitation cross sections for C-C and C-H symmetric stretch modes of acetylene by positron impact. The contribution of these infrared inactive modes to the annihilation parameter is also addressed. The Feshbach projection operator approach was employed to vibrationally resolve e{sup +}-acetylene scattering phase shifts obtained with the Schwinger multichannel method. The present results point out a virtual state pole at the equilibrium geometry of acetylene that becomes a bound state as either bond is stretched, in qualitative agreement with previous calculations for small hydrocarbons. The vibrational couplings are stronger for the C-C mode, giving rise to a bound state pole within the Franck-Condon region of the vibrational ground state. These bound and virtual states give rise to sharp threshold structures (vibrational resonances) in both the vibrational excitation cross sections and the annihilation parameter (Z{sub eff}). We found fair agreement between the present calculations and previously reported e{sup +}-acetylene vibrational excitation cross sections.

Multimode vibrational couplings in resonant positron annihilation

In this work, we show that multimode vibrational couplings in resonant positron annihilation can arise from positron-induced distortions of the potential energy surface.

Positron interactions with molecules

In this work, we report elastic integral cross sections for low energy positron scattering by carbon monoxide and model calculations for vibrationally enhanced positron annhilation on molecules. The former studies employed the Schwinger Multichannel method in the static plus polarization approximation with two different basis sets to access numerical convergence. In the annihilation studies, the role of multimode vibrational couplings was surveyed with the help of an analytically solvable model based on the Feshbach projection operator formalism.

Low Energy Positron Scattering by carbon monoxide

In this work, we present recent calculations of positron scattering by the carbon monoxide molecule. We employed the Schwinger multichannel method to obtain integral and differential cross sections for energies up to 10.0 eV. Basis set convergence was surveyed with the inclusion of one f-type orbital.