Correlation-Polarization Effects in Electron/Positron Scattering from Acetylene: A Comparison of Computational Models

Different computational methods are employed to evaluate elastic (rotationally summed) integral and differential cross sections for low energy (below about 10 eV) positron scattering off gas-phase C$_2$H$_2$ molecules. The computations are carried out at the static and static-plus-polarization levels for describing the interaction forces and the correlation-polarization contributions are found to be an essential component for the correct description of low-energy cross section behavior. The local model potentials derived from density functional theory (DFT) and from the distributed positron model (DPM) are found to produce very high-quality agreement with existing measurements. On the other hand, the less satisfactory agreement between the R-matrix (RM) results and measured data shows the effects of the slow convergence rate of configuration-interaction (CI) expansion methods with respect to the size of the CI-expansion. To contrast the positron scattering findings, results for electron-C$_2$H$_2$ integral and differential cross sections, calculated with both a DFT model potential and the R-matrix method, are compared and analysed around the shape resonance energy region and found to produce better internal agreement

Electron collisions with OClO using the R-matrix method

The R-matrix method is used to calculate elastic and excitation cross sections of four low-lying electronic states of the OClO molecule, designated as the X 2B1, 1 2A1, 1 2B2 and 1 2A2states. Eight states, four doublets and four quartets are included in the close-coupled expansion; these excited states are predicted to have vertical excitation energies in the range 2.956-8.206 eV, in fair agreement with published multireference configuration-interaction calculations. The experimentally determined excitation energy of the atmospherically most important state 1 2A2 centred around 3.5 eV is in excellent agreement with our value of 3.44 eV. A bound state of OClO- with 1A1 symmetry with an adiabatic electron affinity of 1.558 eV at equilibrium geometry of OClO is found. There is a shape resonance of 3B1symmetry at 2.96 eV and higher-lying shape resonances of 1B1, 1A2 and 3A2 symmetries located at 5.75, 8.06 and 7.22 eV, respectively. All the resonances are rather broad. The resonance positions correlate well with the maxima found in dissociative electron attachment cross section measurements. Rotationally inelastic scattering cross sections are compared with experiment and there is very good agreement for electron energies above 100 meV. Excitation cross sections for three excited states are presented for electron-impact energies up to 10 eV. Total integral cross sections are also compared with experiment

Electron collisions with H

The R-matrix approach is used to calculate the elastic differential, integral, and momentum transfer cross sections for electron impact on H2CNH within static-exchange-polarization (SEP) and close-coupling (CC) approximations. The electron-impact excitation cross sections from the ground state to the first three low-lying electronic excited states of H2CNH molecule have been also studied for the first time. A shape resonance of 2A′′ symmetry located at 1.95 eV with a width of 0.56 eV is detected within CC model. The effective collision frequencies over a wide electron temperature range (200−30 000 K) are calculated using the data of the momentum-transfer cross section

Elastic and vibrationally-resolved excitation cross sections for electron impact with MgF radical using R-matrix

We report the calculations of elastic and vibrationally- resolved electronic excitation cross sections by an electron impact on MgF radical by using the R-matrix approach in the 0.1–10 eV energy range. A systematic study considering various basis sets, polarisation and the Born closure technique is performed. For bench mark study, we have used the basis sets like 6-31G*, 6-311G*, DZP and cc-pVTZ to understand the bound state properties of MgF. The final target and hence the scattering calculations are reported using the cc-pVTZ basis set as it yielded the best ground state energy, dipole moment and the energy thresholds of the excited states. Various scattering models like Static Exchange (SE), Static-Exchange-Polarisation (SEP) and Close-Coupling (CC) models are invoked to investigate the role of correlation and polarisation effects on the cross sections. The trial wave function of the entire scattering system (MgF + electron) consists of the ground and many excited states. These target states are constructed by including correlation effects via configuration interaction (CI) approach. Other types of cross sections like differential, momentum transfer, ionisation and elastic rate constants are also reported

Electron-impact cross sections of X

Various electron-impact cross sections are obtained for halogenated derivatives of formaldehyde like H2CO, F2CO, Cl2CO, Br2CO using the Single Centre Expansion method. The molecular wavefunction of the targets are obtained from the multi-centre expansion of the Gaussian-type orbitals within the single determinant Hartree Fock self consistent field scheme. The effects due to static, correlation-polarization and exchange included to model the electron-molecule interactions are approximated by their local nature. The correlation-polarization potential includes short and long range polarization electron dynamical effects. The dipole and higher order multipole terms are considered in the multipole expansion of the target at centre of mass. The coupled scattering equations are solved using Volterra integral form to obtain the elastic cross sections. The inelastic contributions to collision process are approximated by ionization cross sections. The two cross sections are added to obtain the total cross sections from ionization threshold to 5 keV. The collision data generated from this approach are consistent with the available results. The study of scattering from homologous series has helped in proposing a simple empirical formula to estimate the total cross sections for any member of the homologous family

RECENT RADIATIVE AND COLLISIONAL ATOMIC DATA OF ASTROPHYSICAL INTEREST

A l'aide de programmes d'ordinateur sophistiqués et de modèles physiques élaborés, des données atomiques radiatives et collisionnelles précises d'intérêt astrophysique ont été ou sont en train d'être calculées. Les cas traités comprennent les transitions radiatives entre états liés dans les configurations 2p4 et 2s2p5 de nombreux ions de la séquence isoélectronique de l'oxygène, la photoionisation de l'état fondamental du fer neutre, l'excitation par impact électronique des transitions interdites de structure fine dans la configuration fondamentale 3p3 de C2 III, Ar IV et K V, et la production en masse de données radiatives pour les ions des séquences isoélectroniques de l'oxygène et du fluor, en tant que contribution au projet international Opacity.Using sophisticated computer programs and elaborate physical models, accurate radiative and collisional atomic data of astrophysical interest have been or are being calculated. The cases treated include radiative transitions between bound states in the 2p4 and 2s2p5 configurations of many ions in the oxygen isoelectronic sequence, the photoionisation of the ground state of neutral iron, the electron impact excitation of the fine-structure forbidden transitions within the 3p3 ground configuration of C2 III, Ar IV and K V, and the mass-production of radiative data for ions in the oxygen and fluorine isoelectronic sequences, as part of the international Opacity Project