K. L. Baluja

M1 and E2 transition probabilities for states within the 2p4 configuration of the O I isoelectronic sequence

Excitation energies and probabilities for M1 and E2 transitions within the 2p4 ground-state configuration of 17 species in the O I isoelectronic sequence have been calculated using configuration interaction wavefunctions in the computer program CIV3. Relativistic corrections were included in the Breit-Pauli approximation. Correlation effects within and outside the n=2 complex were taken into account. Results are compared with other work. In the case of oxygen, the value of 0.776 s for the lifetime of the 1S0 state agrees well with the experimental value of 0.76 s. For excitation energies in elements up to Z=28, the authors tend to obtain better agreement with measured values than other theoretical work.

Positron collisions with molecular hydrogen: cross sections and annihilation parameters calculated using the R-matrix with pseudo-states method

The molecular R-matrix with pseudo-states (MRMPS) method is employed to study positron collisions with H2. The calculations employ pseudo-continuum orbital sets containing up to h (l = 5) functions. Use of these high l functions is found to give converged eigenphase sums. Below the positronium formation threshold, the calculated cross sections agree with other high-accuracy theories and generally with the measurements. Calculation of the positron annihilation parameter Zeff with the MRMPS wavefunctions gives values significantly higher than other R-matrix wavefunctions but still do not completely converge with h functions. Extrapolation to higher l-values leads to a predicted value of Zeff for H2 of about 10.4. The MRMPS method is both completely general and ab initio; it can therefore be applied to positron collisions with other molecular targets.

Electron-impact study of formaldehyde using the R-matrix method

The R-matrix method, involving eight target states in the close-coupling formalism, is used to calculate elastic integral, differential and momentum transfer cross sections for electron impact on the formaldehyde molecule. We have also obtained the excitation cross sections for the seven lowest-lying electronically excited states which have symmetries 1 3A1, 1 1,3A2, 1 1,3B1 and 1 1,3B2. Their vertical excitation energies from the equilibrium geometry of the ground state X 1A1 lie in the range 3.58–9.99 eV. Configuration interaction (CI) wavefunctions are used to represent the target states. In our CI model, we keep the six core electrons frozen in doubly occupied molecular orbitals 1a1, 2a1 and 3a1. The complete active space consists of ten valence electrons that are allowed to move freely among the eight molecular orbitals: 4a1, 5a1, 6a1, 1b1, 2b1, 1b2, 2b2 and 3b2. In this CI model, we obtain good agreement of the dipole moment of the ground state with the experimental value, and a good representation of the vertical excitation spectrum of the excited states included in our calculation. We have also investigated the electron impact rotationally elastic and rotationally excitation transitions for the ground state for this asymmetric top molecule and obtained the rotationally resolved differential and integral cross sections for energies up to 20 eV. Our calculations do not detect any bound H2CO− states at the equilibrium geometry of the H2CO molecule. We find a shape resonance of the 2B1 symmetry with its resonance position at 1.32 eV and a corresponding resonance width of 0.546 eV at the equilibrium geometry of the molecule. This resonance provides a pathway for dissociative electron attachment when the CO bond is stretched beyond 3a0. Born correction is applied for the elastic and the dipole allowed transitions to account for higher partial waves excluded in the R-matrix calculation. We also compare R-matrix differential, partial, momentum transfer and excitation cross sections with the other work.

Electron collision with the silicon monoxide (SiO) molecule using the R-matrix method

SiO is a molecule that is well known astrophysically. Electron scattering calculations are presented at the static-exchange and close-coupling approximations, one including 24 target states and another including 48 states. Our study predicts the existence of several low-lying narrow 2Π, 2Δ and 2Σ− Feshbach resonances and confirms the existence of a 2Π bound state. Results from the 48-state close-coupling calculation have been employed to calculate rotational (de)excitation rates and rate-fitting coefficients, which are useful in astrophysical modelling. Ionization cross sections, rotationally summed and resolved differential and integral cross sections are also presented.

Elastic and excitation processes of electron impact on C3 using the R-matrix method

R-matrix calculations are carried out for the scattering of electrons from C3 molecules for an incident electron energy range of 1–10 eV. Elastic differential, integral and momentum transfer cross sections are obtained by summing over rotationally elastic and rotationally inelastic cross sections for rotor states up to J = 4. The excitation cross sections from the ground state of C3, at its equilibrium geometry, to its nine low-lying electronically excited states are also presented. We have included 15 states in the close coupling expansion of the scattering system, where each target state is represented by configuration interaction (CI) wavefunctions. The vertical spectrum of energy levels obtained with CI wavefunctions is in good agreement with other theoretical work. We have detected one shape resonance, two Feshbach resonances and five core-excited shape resonances in the energy range 2–7 eV. The energetics of these resonances and their assignments have been given. We also carried out R-matrix calculations in the static-exchange (SE) and static-exchange plus polarization approximations (SEP) to facilitate the recognition of the nature of the various resonances found. There is no other theoretical or experimental work available for the scattering parameters to compare with our work.

Electron scattering by the sulfur fluoride radical using the R-matrix method

The R-matrix method is used to study electron collisions with the molecular radical sulfur fluoride (SF). The elastic and excitation cross sections of the six lowest-lying electronically excited states of the SF radical are presented for incident electron energies up to 15 eV. These excited states have symmetries 4Σ−, 2Σ−, 2Δ, 2Σ+, 2Π and 4Π, and vertical excitation energies in the range 3.32–9.48 eV. Configuration interaction (CI) wavefunctions are used to represent the target states. In our CI model we keep the 1σ, 2σ, 3σ, 4σ and 1Π molecular orbitals fully occupied and the remaining 13 electrons are free to occupy the 5σ, 6σ, 7σ, 8σ, 9σ, 2Π and 3Π orbitals. We detect a bound state of SF− of 1Σ+ symmetry with an adiabatic electron affinity of 1.46 eV. There are two shape resonances of 3Π and 1Π symmetries centred around 1 and 2 eV respectively, with a common configuration 3Π38σ. Our calculations also predict four core-excited shape resonances of symmetries 3Σ−, 1Σ−, 3Π and 1Π in the energy range 8–13 eV having configurations 3Π28σ2 and 2Π33Π48σ.

Electron collisions with an ozone molecule using the R-matrix method

Elastic integral, differential and momentum transfer cross sections for electron collisions with an O3 molecule have been calculated in a 16-state R-matrix approach. The 16 target states have symmetries X1A1, 13B2, 13B1, 13A2, 11B1, 11A2, 23B2, 21A1, 11B2, 23A2, 21A2, 23B1, 21B2, 21B1, 31A1 and 13A1 and have been represented by configuration interaction (CI) wavefunctions. In our CI model, we keep the core 6 electrons frozen in doubly occupied molecular orbitals 1a1, 2a1 and 1b2. The complete active space consists of 18 valence electrons that are allowed to move freely among 12 molecular orbitals: 3a1, 4a1, 5a1, 6a1, 7a1, 1b1, 2b1, 2b2, 3b2, 4b2, 5b2 and 1a2. This CI model gave an adequate description of the vertical spectrum of these excited states which span the energy range 0?10 eV and also gave a good representation of the charge cloud of the ground state at its equilibrium geometry which provided a dipole moment of 0.61 D in good accord with the experimental value 0.53 D. Our calculations detect one bound O?3 state (2B1) at the equilibrium geometry of the O3 molecule. We also find two broad shape resonances in 2A1 and 2B2 symmetries out of which the 2A1 resonance supports dissociative electron attachment when an O?O bond is stretched beyond 3.1 a0. Born correction is applied for the elastic and the dipole allowed transitions to account for partial waves higher than l = 4 that are excluded in the R-matrix calculation. Elastic and excitation cross sections are presented for incident electron energies up to 15 eV and our results are compared with the other theoretical and experimental works.

Electron collisions with methylidyne (CH) radical using the R-matrix method

The R-matrix method is used to calculate the elastic and the excitation cross sections from the ground state X 2Π to the four low-lying electronically excited states a 4Σ-, A 2Δ, B 2Σ- and C 2Σ+ of methylidyne (CH) radical. Configuration interaction (CI) wavefunctions are used to represent the target states. In our CI model we keep the 1σ orbital doubly occupied and the remaining electrons are free to occupy the 2σ,3σ,4σ,1π,2π,3π and 1δ orbitals. This model gives an equilibrium bond length, Re, of X 2Π state equal to 2.113 a0 which is in excellent agreement with the experimental value of 2.116 a0 and a CH equilibrium dipole moment of 1.53 D which is close to the experimental value of 1.46±0.06 D. Scattering calculations are performed in the static-exchange, static-exchange plus polarization, 5-state and 6-state models. Our best 6-state model also includes the D 2Π state. The vertical excitation energies lie in the range 0.32-7.29 eV and agree within three per cent of the experimental values. We find a bound state of CH- of 3Σ- symmetry with an electron affinity of 0.61 eV at Re. Below 1 eV there are shape resonances in 1Σ+ and 1Δ symmetries. Both of these resonances have the configuration 3σ21π2. Born correction is applied for dipole-allowed transitions to account for higher partial waves excluded in the R-matrix calculation. Cross sections are given for scattering energies up to 10 eV.

Excitation energies and oscillator strengths for the allowed transitions 2p4 3P to 2s2p5 3P0 and 2p4 1D, 1S to 2s2p5 1P0 in the O I isoelectronic sequence

Excitation energies and oscillator strengths for the allowed transitions 2p4 3P to 2s2p5 3P0, 2p4 1D to 2s2p5 1P0 and 2p4 1S to 2s2p5 1P0 have been calculated in LS coupling for 17 species in the oxygen isoelectronic sequence (from O I to Kr XXIX), using configuration interaction wavefunctions in the computer program CIV3. The configurations included represent the internal-, semi-internal- and all-external-type correlations. Excitation energies were also calculated while including the first-order relativistic corrections. With these corrections, the disagreement between calculated and experimental values is less than 1% for two-thirds of the transitions studied. The maximum difference is 2.6% for transition 1S to 1P0 in Ne III. The present oscillator strengths in the length and velocity formulations are compared with previous work.

Total cross-sections for electron scattering from iron at 10–5000 eV using a model optical potential

We report calculations on the total (elastic plus inelastic) electron-scattering cross sections in the energy range 10–5000 eV. A model complex optical potential, composed of static, exchange, polarisation and absorption terms, is employed to describe the collision system at each electron energy. The Iron atom is described by Dirac-Hartree-Fock-Slater self-consistent charge density. The complex phase shifts are computed in a variable phase approach. The absorption cross sections are compared with the experimental results. The experimental absorption cross sections are obtained by adding the experimental ionisation cross sections and available experimental excitation cross sections for electron impact of the allowed transitions a5D → (x,y,z)5D0, (w,y,z)5P0. We have good qualitative agreement between our results and the experimental results available below 200 eV. The Born-Bethe parameters are also calculated. Elastic differential cross-sections with and without absorption are also reported at a few selected energies.