M. A. P. Lima

Cross sections and photoelectron asymmetry parameters for photoionization of H2O

The iterative Schwinger variational method is used to obtain cross sections and photoelectron asymmetry parameters for photoionization of the three outermost valence orbitals (1b1, 3a1, and 1b2) of H2O for photon energies from near threshold to 50 eV. A comparison of these calculated results with available experimental data is encouraging.

Studies of the photoionization cross sections of CH4

We present cross sections and asymmetry parameters for photoionization of the 1t_2 orbital of CH_4 using static‐exchange continuum orbitals of CH^+_4 to represent the photoelectron wave function. The calculations are done in the fixed‐nuclei approximation at a single internuclear geometry. To approximate the near‐threshold behavior of these cross sections, we assumed that the photoelectron spectrum is a composite of three electronic bands associated with the Jahn–Teller components of the distorted ion. The resulting cross sections reproduce the sharp rise seen at threshold in the experimental data and are in good agreement with experiment at higher energy. The agreement between the calculated and measured photoelectron asymmetry parameters is, however, less satisfactory.

Electron collisions with the HCOOH ··· (H2O)n complexes (n = 1, 2) in liquid phase: The influence of microsolvation on the π ∗ resonance of formic acid

We report momentum transfer cross sections for elastic collisions of low-energy electrons with the HCOOH···(H2O)n complexes, with n = 1, 2, in liquid phase. The scattering cross sections were computed using the Schwinger multichannel method with pseudopotentials in the static-exchange and static-exchange plus polarization approximations, for energies ranging from 0.5 eV to 6 eV. We considered ten different structures of HCOOH···H2O and six structures of HCOOH···(H2O)2 which were generated using classical Monte Carlo simulations of formic acid in aqueous solution at normal conditions of temperature and pressure. The aim of this work is to investigate the influence of microsolvation on the π* shape resonance of formic acid. Previous theoretical and experimental studies reported a π* shape resonance for HCOOH at around 1.9 eV. This resonance can be either more stable or less stable in comparison to the isolated molecule depending on the complex structure and the water role played in the hydrogen bond interaction. This behavior is explained in terms of (i) the polarization of the formic acid molecule due to the water molecules and (ii) the net charge of the solute. The proton donor or acceptor character of the water molecules in the hydrogen bond is important for understanding the stabilization versus destabilization of the π* resonances in the complexes. Our results indicate that the surrounding water molecules may affect the lifetime of the π* resonance and hence the processes driven by this anion state, such as the dissociative electron attachment.

Cross sections for collisions of low‐energy electrons with the hydrides PH3, AsH3, SbH3, SnH4, TeH2, and HI

We calculated integral and differential cross sections for scattering of low‐energy electrons by two groups of hydrides from 10 to 30 eV. The first group is composed by the hydrides of elements in the same column of the Periodic Table and includes PH3, AsH3, and SbH3. The second group is formed by hydrides in the same row and includes SnH4, SbH3, TeH2, and HI. The calculations employed the Schwinger multichannel method with norm‐conserving pseudopotentials [M.H.F. Bettega, L.G. Ferreira, and M.A.P. Lima, Phys. Rev. A 47, 1111 (1993)]. Our goal is to find similarities and differences in the cross sections in these two groups.

Elastic scattering of low-energy electrons by OCS molecules.

We report results from an ab initio calculation of low-energy electron scattering by OCS molecules. We used the Schwinger multichannel method with pseudopotentials at the fixed-nuclei static-exchange approximation to calculate elastic integral, differential and momentum transfer cross sections in the energy range from 5 to 50 eV. We compare our results with available theoretical results and experimental data. Through the symmetry decomposition of our integral cross section and eigenphase sum analysis, we found structures in the cross sections that may be interpreted as shape resonances for ∑, ∏ and Δ symmetries. We compared the results for OCS with our previous results on the e––CS2 collision. In particular, we found a similar behaviour in the shape of the symmetry decomposed cross sections of OCS and of CS2 when, for the latter, we sum over the ‘g’ and ‘u’ contributions.

Electron scattering from molecules: applications of the Schwinger multichannel method to e−–CO and e−–C2H4 collisions

To illustrate our recent efforts to obtain electronic excitation cross sections of molecules by electron impact, we present in this paper results for the X 1Σ a 3Π and A 1Π transitions of CO obtained with the Schwinger multichannel method. Our results are in good agreement with other theoretical calculations, although not so good when compared with experiments. We also discuss the importance of inclusion of polarization effects to obtain electronic excitation cross sections of some molecules through an example using the C2H4 molecule, which has a triplet state with a low-energy threshold. Finally, we present a very simple rule to estimate integral electronic excitation cross sections using the differential cross section (DCS) at 900, which can be useful to experimentalists using apparatus with difficulties to measure the DCSs at angles around 0 and 180 degrees. We show its efficiency for the present electronic excitation of the C2H4 molecule by electron impact.

Low-energy electron scattering by N, P, As and Sb

We report elastic integral, momentum transfer and differential cross sections from 10-30 eV for electron scattering by (X = N, P, As, Sb). These results were obtained at the static-exchange approximation with the Schwinger Multichannel Method with Pseudopotentials [M H F Bettega, L G Ferreira and M A P Lima 1993 Phys. Rev. A 47 1111]. Our results for are in good agreement with experimental data. We also compare our results with previous calculations on (X = P, As, Sb) [M H F Bettega, M A P Lima and L G Ferreira 1996 J. Chem. Phys. 105 1029] and found, as expected, that the cross sections are larger than the corresponding cross sections.

Low Energy Electron Scattering from Fuels

In order to understand and optimize processes occurring during the ignition of plasma and its consequences in post-discharge for an internal combustion engine, especially considering the spark plug, we have produced in this work some basic information necessary to modeling spark ignition in alcohol-fuelled engines. Total cross sections of electron scattering by methanol and ethanol molecules in the energy range from 60 to 500 eV are reported, using the linear transmission method based on the Beer-Lambert law to first approximation. Additionally to that, measurements and calculations of differential cross sections for elastic low-energy (rotationally unresolved) electron scattering were also discussed, for impact energies of 1, 2, 5, 10, 15, 20, 30, 50, and 100 eV and for scattering angles of 5°–130°. The measurements were obtained using the relative flow method with an aperture source, and calculations using two different implementations of the Schwinger multichannel method, one that takes all electrons into account and is adapted for parallel computers, and another that uses pseudopotentials and considers only the valence electrons.

Cross sections for below threshold electron excitation of furan

Experimental and theoretical cross sections are present for electron impact excitation of the 3B2 and 3A1 electronic states of furan by sub 20eV electrons.