Marco A. P. Lima

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 ...

Note on the generation of Gaussian bases for pseudopotential calculations

We present a technique to generate Cartesian Gaussian bases for electronic configuration and cross-section calculations on molecules. The technique is specially useful for pseudopotential work, when the bases cannot be tabulated because they depend on the specific choice of the pseudopotential.

Studies of electron collisions with polyatomic molecules using distributed‐memory parallel computers

Elastic electron scattering cross sections from 5–30 eV are reported for the molecules C2H4, C2H6, C3H8, Si2H6, and GeH4, obtained using an implementation of the Schwinger multichannel method for distributed‐memory parallel computer architectures. These results, obtained within the static‐exchange approximation, are in generally good agreement with the available experimental data. These calculations demonstrate the potential of highly parallel computation in the study of collisions between low‐energy electrons and polyatomic gases. The computational methodology discussed is also directly applicable to the calculation of elastic cross sections at higher levels of approximation (target polarization) and of electronic excitation cross sections.

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)].

An experimental and theoretical investigation into the excited electronic states of phenol

We present experimental electron-energy loss spectra (EELS) that were measured at impact energies of 20 and 30 eV and at angles of 90° and 10°, respectively, with energy resolution ∼70 meV. EELS for 250 eV incident electron energy over a range of angles between 3° and 50° have also been measured at a moderate energy resolution (∼0.9 eV). The latter spectra were used to derive differential cross sections and generalised oscillator strengths (GOS) for the dipole-allowed electronic transitions, through normalization to data for elastic electron scattering from benzene. Theoretical calculations were performed using time-dependent density functional theory and single-excitation configuration interaction methods. These calculations were used to assign the experimentally measured spectra. Calculated optical oscillator strengths were also compared to those derived from the GOS data. This provides the first investigation of all singlet and triplet excited electronic states of phenol up to the first ionization potential.

Studies of electron-molecule collisions - Applications to e-H2O

We report elastic differential and momentum transfer cross sections for the elastic scattering of electrons by H2O for collision energies from 2 to 20 eV. These fixed-nuclei static-exchange cross sections were obtained using the Schwinger variational approach. In these studies the exchange potential is directly evaluated and not approximated by local models. The calculated differential cross sections, obtained with a basis set expansion of the scattering wave function, agree well with available experimental data at intermediate and larger angles. As used here, the results cannot adequately describe the divergent cross sections at small angles. An interesting feature of the calculated cross sections, particularly at 15 and 20 eV, is their significant backward peaking. This peaking occurs in the experimentally inaccessible region beyond a scattering angle of 120°. The implication of this feature for the determination of momentum transfer cross sections is described.

Electron-impact excitation of H2: minimal orbital basis for single configuration interaction

We report theoretical differential excitation cross sections for scattering of electrons by H2 molecules using the Schwinger multichannel approach (SMC). This study differs from previous applications of the SMC method in the description of the excited states (which are now obtained through the single configuration interaction technique) and in the level of the multichannel coupling. The calculation is performed with singlet and triplet states present in the manifold. The results given by this strategy show a significant improvement to experimental data in comparison with earlier two-state close coupling calculations.

Differential cross sections for electron impact excitation of the electronic bands of phenol

We report results from a joint theoretical and experimental investigation into electron scattering from the important organic species phenol (C6H5OH). Specifically, differential cross sections (DCSs) have been measured and calculated for the electron-impact excitation of the electronic states of C6H5OH. The measurements were carried out at energies in the range 15-40 eV, and for scattered-electron angles between 10° and 90°. The energy resolution of those experiments was typically ∼80 meV. Corresponding Schwinger multichannel method with pseudo-potentials calculations, with and without Born-closure, were also performed for a sub-set of the excited electronic-states that were accessed in the measurements. Those calculations were conducted at the static exchange plus polarisation (SEP)-level using a minimum orbital basis for single configuration interaction (MOBSCI) approach. Agreement between the measured and calculated DCSs was typically fair, although to obtain quantitative accord, the theory would need to incorporate even more channels into the MOBSCI.

Cross sections for electron-impact excitation of the H2 molecule using the MOB-SCI strategy

In this paper, we report integral and differential cross sections for the electronic excitation of H2 molecules by electron-impact. Our scattering amplitudes were calculated using the Schwinger multichannel method within the minimal orbital basis for single configuration interactions (MOB-SCI) level of approximation. Through the use of the present strategy we have investigated the coupling effects among ground state and first singlet and triplet states of the same spatial symmetry. The five-state (nine for degenerated states) close-coupling calculations joined the advantages of a well-described set of physical states of interest with a minimum associated pseudo-state space. The results obtained by means of the MOB-SCI technique show a significant improvement towards experimental data in comparison with previous two-channel close-coupling calculations.