Eliane M. de Oliveira

Electron collisions with phenol: Total, integral, differential, and momentum transfer cross sections and the role of multichannel coupling effects on the elastic channel

We report theoretical and experimental total cross sections for electron scattering by phenol (C6H5OH). The experimental data were obtained with an apparatus based in Madrid and the calculated cross sections with two different methodologies, the independent atom method with screening corrected additivity rule (IAM-SCAR), and the Schwinger multichannel method with pseudopotentials (SMCPP). The SMCPP method in the Nopen-channel coupling scheme, at the static-exchange-plus-polarization approximation, is employed to calculate the scattering amplitudes at impact energies ranging from 5.0 eV to 50 eV. We discuss the multichannel coupling effects in the calculated cross sections, in particular how the number of excited states included in the open-channel space impacts upon the convergence of the elastic cross sections at higher collision energies. The IAM-SCAR approach was also used to obtain the elastic differential cross sections (DCSs) and for correcting the experimental total cross sections for the so-called forward angle scattering effect. We found a very good agreement between our SMCPP theoretical differential, integral, and momentum transfer cross sections and experimental data for benzene (a molecule differing from phenol by replacing a hydrogen atom in benzene with a hydroxyl group). Although some discrepancies were found for lower energies, the agreement between the SMCPP data and the DCSs obtained with the IAM-SCAR method improves, as expected, as the impact energy increases. We also have a good agreement among the present SMCPP calculated total cross section (which includes elastic, 32 inelastic electronic excitation processes and ionization contributions, the latter estimated with the binary-encounter-Bethe model), the IAM-SCAR total cross section, and the experimental data when the latter is corrected for the forward angle scattering effect [Fuss et al., Phys. Rev. A 88, 042702 (2013)].

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.

Communication: Transient anion states of phenol…(H2O)n (n = 1, 2) complexes: Search for microsolvation signatures

We report on the shape resonance spectra of phenol-water clusters, as obtained from elastic electron scattering calculations. Our results, along with virtual orbital analysis, indicate that the well-known indirect mechanism for hydrogen elimination in the gas phase is significantly impacted on by microsolvation, due to the competition between vibronic couplings on the solute and solvent molecules. This fact suggests how relevant the solvation effects could be for the electron-driven damage of biomolecules and the biomass delignification [E. M. de Oliveira et al., Phys. Rev. A 86, 020701(R) (2012)]. We also discuss microsolvation signatures in the differential cross sections that could help to identify the solvated complexes and access the composition of gaseous admixtures of these species.

Associated factors with discontinuation use of combined oral contraceptives

PURPOSE Due to the scarce information available in Brazil in relation to the number of women who initiated the use of combined oral contraceptives and prematurely discontinued, the objective was to assess the reasons for discontinuation of the use of several combined oral contraceptives among Brazilian women living in urban areas. METHODS A cross-sectional study with 400 gynecologists registered withy the Brazilian Federation of Obstetricians and Gynecologists. Each physician interviewed 10 non-pregnant, not breastfeeding, not amenorrheic women aged 18 to 39 years who consulted requesting combined oral contraceptive (COC) with a questionnaire at the beginning of use and at six months later. The questionnaire included sociodemographic data, type of COC chosen or prescribed and reasons for discontinuation when it occurred during follow-up. The strategy of selection allowed the inclusion of women from different socioeconomic strata, however, only those attended at private or insurance offices. The sample size was estimated at 1,427 women. RESULTS A total of 3,465 interviews were conducted at the first visit and 1,699 six months later. The women were 20 to 29 years old, 57.3% were single and an equal proportion of 45.0% attended high school or college. Most (60.7%) were nulligravidas and among those who had used some contraceptive before, 71.8% had used a COC. Among the more prescribed or chosen COC the most prevalent were monophasic with ethynil estradiol (20 µg) and regarding progestin the most prevalent was with gestodene (36.5%) followed by a COC with drosperinone (22.0%). At six months 63.5% still used COC. Among those who discontinued the main reasons were wishing to become pregnant (36.5%) and side effects (57.3%) and the most prevalent were headache (37.6%), weight gain (16.6%) and irregular bleeding (23.6%). CONCLUSIONS The continuation rate of COC was low at six months and this study could contribute to a better counseling on the part of physicians of patients who initiate COC about side-events that are rare, minimal and temporary and about the benefits of COC use.

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.

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.

Transient ions in electron and positron scattering

We report on recent advances in studies of transient ions formed in electron and positron scattering by molecules. We briefly discuss elastic electron collisions against pyrrole and glycine, as well as electron affinities of glycine-water clusters. Positron scattering and annihilation on small molecules is also discussed.

Low-energy electron scattering by pyrrole

We present elastic cross sections for electron-pyrrole scattering obtained with the Schwinger Multichannel Method. For this C2v system, we found three shape resonances in the A1, B2 and A2 irreducible representations that could give rise to direct and indirect mechanisms for dissociative electron attachment.