Márcio T. do N. Varella

LOW-ENERGY ELECTRON SCATTERING BY H2O, H2S, H2SE, AND H2TE

We report elastic differential, integral, and momentum transfer cross-sections for H2X molecules (X: O, S, Se, and Te) obtained at the static exchange level of approximation. The energy range considered was from 2 up to 30 eV for H2O and from 5 up to 30 eV for the other molecules. Our calculations were performed with the Schwinger multichannel method with pseudopotentials [M. H. F. Bettega, L. G. Ferreira, and M. A. P. Lima, Phys. Rev. A 47, 1111 (1993)], combined with a Born closure procedure in order to account for the long-range potential due to the permanent dipole moment of the targets. Our calculated cross-sections for H2O and H2S are in good agreement with other theoretical results. Agreement with available experimental data is also encouraging. It was found that molecular size plays a crucial role in the scattering process. The influence of heavy and H atoms in the collisions is also discussed. For the integral cross-sections of the heavier molecules we also investigated incident energies below 5 ...

Progress with the Schwinger multichannel method in positron- molecule scattering

We report calculations for low-energy positron‐C2H2 and ‐N2 scattering obtained through the Schwinger multichannel method (SMC). Integral and diAerential cross-sections and the annihilation parameter, Zeff , are presented. For nitrogen, evidence of a Ramsauer‐Townsend minimum and the possibility of overcorrelation in the scattering configuration space are discussed. Annihilation probability densities (APD) calculated for He and H2 seem to bring indirect support to a model previously proposed to explain why acetylene, unlike nitrogen, presents very high low-energy annihilation rates. ” 2000 Elsevier Science B.V. All rights reserved.

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.

Time-resolved photoelectron spectroscopy of proton transfer in the ground state of chloromalonaldehyde: wave-packet dynamics on effective potential surfaces of reduced dimensionality.

We report on a simple but widely useful method for obtaining time-independent potential surfaces of reduced dimensionality wherein the coupling between reaction and substrate modes is embedded by averaging over an ensemble of classical trajectories. While these classically averaged potentials with their reduced dimensionality should be useful whenever a separation between reaction and substrate modes is meaningful, their use brings about significant simplification in studies of time-resolved photoelectron spectra in polyatomic systems where full-dimensional studies of skeletal and photoelectron dynamics can be prohibitive. Here we report on the use of these effective potentials in the studies of dump-probe photoelectron spectra of intramolecular proton transfer in chloromalonaldehyde. In these applications the effective potentials should provide a more realistic description of proton-substrate couplings than the sudden or adiabatic approximations commonly employed in studies of proton transfer. The resulting time-dependent photoelectron signals, obtained here assuming a constant value of the photoelectron matrix element for ionization of the wave packet, are seen to track the proton transfer.

Cross sections for rotational excitations of NH3, PH3, AsH3, and SbH3 by electron impact

We report elastic and rotationally inelastic cross sections for e−–XH3 collisions (X: N, P, As, Sb) at the static-exchange level of approximation. The energy range was from 7.5 up to 30 eV. Our fixed-nuclei scattering amplitudes were obtained through the Schwinger multichannel method with pseudopotentials (SMCPP) [M. H. F. Bettega, L. G. Ferreira, and M. A. P. Lima, Phys. Rev. A 47, 1111 (1993)]. The rotational cross sections were obtained with the help of the adiabatic-nuclei-rotation approximation. There is good agreement with available experimental elastic cross sections. In order to improve rotational cross sections at small scattering angles for the dipole-allowed 00→10 rotational excitation, we have combined the SMCPP and the first Born approximation of the full interaction potential and also of the dipole moment potential. To our knowledge this is the first time that rotational excitation cross sections for these molecules are reported.

The electron-furfural scattering dynamics for 63 energetically open electronic states

We report on integral-, momentum transfer- and differential cross sections for elastic and electronically inelastic electron collisions with furfural (C5H4O2). The calculations were performed with two different theoretical methodologies, the Schwinger multichannel method with pseudopotentials (SMCPP) and the independent atom method with screening corrected additivity rule (IAM-SCAR) that now incorporates a further interference (I) term. The SMCPP with N energetically open electronic states (N(open)) at either the static-exchange (N(open) ch-SE) or the static-exchange-plus-polarisation (N(open) ch-SEP) approximation was employed to calculate the scattering amplitudes at impact energies lying between 5 eV and 50 eV, using a channel coupling scheme that ranges from the 1ch-SEP up to the 63ch-SE level of approximation depending on the energy considered. For elastic scattering, we found very good overall agreement at higher energies among our SMCPP cross sections, our IAM-SCAR+I cross sections and the experimental data for furan (a molecule that differs from furfural only by the substitution of a hydrogen atom in furan with an aldehyde functional group). This is a good indication that our elastic cross sections are converged with respect to the multichannel coupling effect for most of the investigated intermediate energies. However, although the present application represents the most sophisticated calculation performed with the SMCPP method thus far, the inelastic cross sections, even for the low lying energy states, are still not completely converged for intermediate and higher energies. We discuss possible reasons leading to this discrepancy and point out what further steps need to be undertaken in order to improve the agreement between the calculated and measured cross sections.

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.

Calculation of positron binding energies using the generalized any particle propagator theory

We recently extended the electron propagator theory to any type of quantum species based in the framework of the Any-Particle Molecular Orbital (APMO) approach [J. Romero, E. Posada, R. Flores-Moreno, and A. Reyes, J. Chem. Phys. 137, 074105 (2012)]. The generalized any particle molecular orbital propagator theory (APMO/PT) was implemented in its quasiparticle second order version in the LOWDIN code and was applied to calculate nuclear quantum effects in electron binding energies and proton binding energies in molecular systems [M. Díaz-Tinoco, J. Romero, J. V. Ortiz, A. Reyes, and R. Flores-Moreno, J. Chem. Phys. 138, 194108 (2013)]. In this work, we present the derivation of third order quasiparticle APMO/PT methods and we apply them to calculate positron binding energies (PBEs) of atoms and molecules. We calculated the PBEs of anions and some diatomic molecules using the second order, third order, and renormalized third order quasiparticle APMO/PT approaches and compared our results with those previously calculated employing configuration interaction (CI), explicitly correlated and quantum Montecarlo methodologies. We found that renormalized APMO/PT methods can achieve accuracies of ~0.35 eV for anionic systems, compared to Full-CI results, and provide a quantitative description of positron binding to anionic and highly polar species. Third order APMO/PT approaches display considerable potential to study positron binding to large molecules because of the fifth power scaling with respect to the number of basis sets. In this regard, we present additional PBE calculations of some small polar organic molecules, amino acids and DNA nucleobases. We complement our numerical assessment with formal and numerical analyses of the treatment of electron-positron correlation within the quasiparticle propagator approach.

Real-time observation of intramolecular proton transfer in the electronic ground state of chloromalonaldehyde: An ab initio study of time-resolved photoelectron spectra

The authors report on studies of time-resolved photoelectron spectra of intramolecular proton transfer in the ground state of chloromalonaldehyde, employing ab initio photoionization matrix elements and effective potential surfaces of reduced dimensionality, wherein the couplings of proton motion to the other molecular vibrational modes are embedded by averaging over classical trajectories. In the simulations, population is transferred from the vibrational ground state to vibrationally hot wave packets by pumping to an excited electronic state and dumping with a time-delayed pulse. These pump-dump-probe simulations demonstrate that the time-resolved photoelectron spectra track proton transfer in the electronic ground state well and, furthermore, that the geometry dependence of the matrix elements enhances the tracking compared with signals obtained with the Condon approximation. Photoelectron kinetic energy distributions arising from wave packets localized in different basins are also distinguishable and could be understood, as expected, on the basis of the strength of the optical couplings in different regions of the ground state potential surface and the Franck-Condon overlaps of the ground state wave packets with the vibrational eigenstates of the ion potential surface.