T. C. Freitas

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.

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.

A física do violino

In this work we present a general description of the physics of the violin. We examine the concepts which provide the physical support and reveal the richness and the pedagogical potential of the subject. We remark the contributions of physicists such as Helmholtz, Savart, Raman and Saunders to the undesrtanding of the way in which a bowed string vibrates, and the comprehension of the acoustical properties of the instrument. The role of each component of the violin is described in details. We also discuss the importance of the bridge, the plates and the body normal modes of vibration for the acoustical response of the instrument. The air-resonance of the enclosed air in the violin body (Helmholtz resonance) is disscussed using the equivalent fomalism between mechanical, electrical and acoustic oscillating systems. The production of the characteristic sound of the violin, which results from the vibrational waveform of a bowed string, is also described.

Resonances in electron collisions with C2H2Cl2 isomers

We present elastic cross sections for low-energy electron collisions with the isomers of C2H2Cl2 molecule, namely 1,1-dichloroethylene, cis-1,2-dichloroethylene and trans-1,2-dichloroethylene. To compute the cross sections, we employed the Schwinger multichannel method with pseudopotentials in the static exchange and in the static-exchange plus polarization approximations for energies up to 40 eV. We found π* and σ* shape resonances in the cross sections of the three isomers, with positions in relative agreement with the results reported by Burrow et al (1981 Chem. Phys. Lett. 82 270) for the vertical attachment energies. We also carried out electronic structure calculations in order to help in the characterization of these resonances. We found a Ramsauer–Townsend minimum in the cross section of trans-1,2-dichloroethylene at 0.1 eV.

Low-energy electron collisions with proline and pyrrolidine: A comparative study

We present a comparative study on the calculated cross sections obtained for the elastic collisions of low-energy electrons with the amino acid proline (C5H9NO2) and its building block pyrrolidine (C4H9N). We employed the Schwinger multichannel method implemented with pseudopotentials to compute integral, differential, and momentum transfer cross sections in the static-exchange plus polarization approximation, for energies up to 15 eV. We report three shape resonances for proline at around 1.7 eV, 6.8 eV, and 10 eV and two shape resonances for pyrrolidine centered at 7 eV and 10.2 eV. The present resonance energies are compared with available experimental data on vertical attachment energies and dissociative electron attachment, where a good agreement is found. From the comparison of the present results with available calculated cross sections for the simplest carboxylic acid, formic acid (HCOOH), and from electronic structure calculations, we found that the first resonance of proline, at 1.7 eV, is due the presence of the carboxylic group, whereas the other two structures, at 6.8 eV and 10 eV, clearly arise from the pyrrolidine ring. A comparison between the differential cross sections for proline and pyrrolidine at some selected energies of the incident electron is also reported in this paper.

Halogenation effects on electron collisions with CF3Cl, CF2Cl2, and CFCl3

We report differential and integral elastic cross sections for low-energy electron collisions with CF3Cl, CF2Cl2, and CFCl3 molecules for energies ranging from 0.1 eV to 30 eV. The calculations were performed using the Schwinger multichannel method with pseudopotentials in the static-exchange and static-exchange plus polarization approximations. The influence of the permanent electric dipole moment on the cross sections was included using the Born closure scheme. A very good agreement between our calculations and the experimental results of Jones [J. Chem. Phys. 84, 813 (1986)], Mann and Linder [J. Phys. B 25, 1621 (1992); 25, 1633 (1992)] and Hoshino et al. [J. Chem. Phys. 138, 214305 (2013)] was found. We also compare our results with the calculations of Beyer et al. [Chem. Phys. 255, 1 (2000)] using the R-matrix method, where we find good agreement with respect to the location of the resonances, and with the calculations of Hoshino et al. using the independent atom method with screening corrected additivity rule, where we find qualitative agreement at energies above 20 eV. Additional electronic structure calculations were carried out in order to help in the interpretation of the scattering results. The stabilization the lowest σ(∗) resonance due to the exchange of fluorine by chlorine atoms (halogenation effect) follows a simple linear relation with the energy of the lowest unoccupied molecular orbitals and can be considered as a signature of the halogenation effect.

Sinos: Física e música fundidas em bronze

Os sinos sao instrumentos de percussao, da familia dos idiofonos. Sao dispositivos de producao de som apenas aparentemente simples. O seu corpo e constituido de uma liga metalica chamada bronze, que e uma combinacao de cobre e estanho em proporcoes que podem sofrer ligeiras variacoes. No interior do sino encontra-se o badalo - um pendulo que, quando oscila, atinge o corpo fazendo com que o sino soe. A fundicao de um sino e bastante complexa e envolve um grande numero de procedimentos, desde a construcao dos moldes ate a metalurgia e afinacao. O sino moderno sofreu apenas algumas mudancas na sua forma em comparacao aos primeiros sinos fundidos do Seculo IX. Do ponto de vista da acustica existem dificuldades para a sua modelagem, tanto devido aos metodos matematicos envolvidos quanto a obtencao de resultados experimentais, ja que o tamanho exagerado impede o seu translado para um laboratorio. Neste artigo trataremos brevemente da origem, historia, e utilizacao do sino, bem como dos principais processos de fundicao empregados na sua fabricacao e da descricao fisica de como o sino produz som.

Electron collisions with the HCOOH... H2O complex

In this conference we will present cross sections for elastic electron collisions with the CH2O-H2O complex bonded through hydrogen bond. We will investigate electron collisions with different structures of this complex which were obtained by Classical Monte Carlo simulations. This work would help in understanding the hole of water in the dissociative electron attachment in biological molecules.