S S Tayal

R-matrix calculation with non-orthogonal orbitals for electron-impact excitation of atomic oxygen

We have investigated cross sections for the excitation of the 2p33s 5So, 3So, 2p33p 5P, 3P, 2p33d 3Do, 2p33s 3Do, 2p33s 3Po and 2s2p5 3Po states by electron impact using a modified R-matrix method based on the B-spline representation of the scattering orbitals in the energy region from threshold to 100 eV. The 26 spectroscopic bound and autoionizing states of atomic oxygen have been included in the close-coupling expansion. We have also carried out calculations in 8 and 16 state close-coupling approximations and in the Born approximation. We have obtained an accurate representation of the target wavefunctions on the basis of non-orthogonal orbitals, and we compare the present results with other calculations and experiments. Significant discrepancies in the shape and magnitude of cross sections for some of the transitions are noted.

Low-energy electron collisions with atomic oxygen: R-matrix calculation with non-orthogonal orbitals

The low-energy electron collision with neutral oxygen has been investigated using a modified R-matrix method based on the B-spline representation of the scattering orbitals. Integral cross sections for elastic scattering and for excitation of the 3P-1D, 3P-1S and 1D-1S transitions are presented in the energy region from threshold to 30 eV. Accurate representation of target wavefunctions has been obtained on the basis of the non-orthogonal orbitals. Both the correlation and relaxation effects are shown to be important. The close-coupling expansion consists of the 26 spectroscopic bound and autoionizing target states. A calculation with 26 spectroscopic states and 17 pseudo-states has also been carried out to check the effect of coupling to the continuum. The present results for the elastic scattering are in excellent agreement with the measurements, but large discrepancies with the existing experimental data are found for the 3P-1D and 3P-1S transitions. The excitation cross sections agree well with the recent R-matrix calculation with pseudo-states by Thomas et al (Thomas M R J, Bell K L and Berrington K A 1997 J. Phys. B: At. Mol. Opt. Phys. 30 4599) except for lower energies about 5-7 eV of threshold.

B-spline R-matrix with pseudostates approach for electron impact excitation of atomic nitrogen

The B-spline R-matrix with pseudostates approach has been used to calculate excitation cross sections for the forbidden 2s22p34So–2Do, 2Po, 2s22p32Do–2Po and resonance 2s22p34So–2s22p23s4P, 2s2p44P, 2s22p24s4P and 2s22p23d4P transitions in atomic nitrogen for incident electron energies from threshold to 120 eV. The excitation of these transitions gives rise to prominent lines in the spectra of solar and planetary atmospheres. The 24 spectroscopic bound and autoionizing states together with 15 pseudostates are included in the close-coupling expansion. The pseudostates are chosen to approximate the loss of flux into the infinite number of bound and continuum states that are dipole coupled with ground configuration terms. The contribution of the ionization continuum is significant for resonance transitions. An accurate description of target wavefunctions has been obtained on the basis of non-orthogonal spectroscopic and pseudo-orbitals to adequately account for the correlation corrections and interactions. A comparison of calculated cross sections with the measured absolute direct excitation cross sections is presented. A good agreement with measured integral cross sections is noted except at 5 eV for the forbidden transition and 30 eV for the resonance transitions.

Electron impact collision strengths and rates for neutral sulphur using the B-spline R-matrix approach

Our previous calculations based on the use of the B-spline basis in the R-matrix approach for low-energy electron collisions with atomic sulphur (Zatsarinny O and Tayal S S 2001 J. Phys. B: At. Mol. Opt. Phys. 34 3383) have been extended to determine electron collisional excitation strengths and rates for transitions between the 3s23p4 3P, 1D and 1S LS states and from these states to the 24 states of the excited configurations. The close-coupling expansion consists of the 27 spectroscopic bound and autoionizing target states. The collision strengths are represented by simple parametric functions of scaled energy. Their values in a very wide energy range are easily obtained using the tabulated parameters. The same parameters can be used to determine effective collision strengths over a wide electron temperature range. Branching ratios are presented that can be used to estimate cascade contributions and to determine emission cross sections for transitions between low-lying states.

Electron-impact studies of atomic oxygen: I. Differential and integral cross sections; experiment and theory

We report both experimental and theoretical differential and integral excitation cross sections of atomic oxygen corresponding to the 2s22p4 3P→3s 3S (130.4 nm), 2s22p4 3P→3d 3D (102.7 nm), 2s22p4 3P→3s 3D (98.9 nm) and 2s22p4 3P→3s 3P (87.8 nm) transitions at 30, 50, and 100 eV electron-impact energies. Experimental measurements have been made in the angular range from 0° to 25° with a conventional electrostatic electron energy-loss spectrometer. The atomic O differential cross sections (DCSs) were put on an absolute scale by normalization to the O2 DCS values of Johnson and Kanik (2001). Extrapolation of the measured results to larger angles was performed using theoretical calculations as a guide, and integral cross sections were derived. Theoretical calculations based on the R-matrix method, along with other available experimental data, have been compared with the current experimental results.

Oscillator strengths of electric dipole transitions in S3

Oscillator strengths and transition probabilities of electric dipole transitions among the fine-structure levels of the 3s23p, 3s3p2, 3s23d, 3s24s, 3p3, 3s3p3d, 3s24p, 3s24d, 3s24f and 3s3p4s configurations and of spin-forbidden electric dipole transitions among the levels of the 3s23p, 3s3p2 and 3s23d configurations of S3+ are calculated using fairly extensive configuration-interaction wavefunctions. The relativistic corrections are included through the Breit-Pauli Hamiltonian. The calculated excited energies are in close agreement with measurement. Small adjustments to the diagonal elements of the Hamiltonian matrices are made to further bring calculated energy separations between levels as close as possible to the measured values. The present results are compared with other available calculations and experiments and significant discrepancies are noted for some transitions.

Autoionizing resonances in the photoionization of ground state atomic magnesium

Photoionization of the ground state 1s22s22p63s2 1S of atomic magnesium is studied theoretically in the energy region between the Mg+(3s) and Mg+(4p) thresholds using the non-iterative variational R-matrix method combined with multichannel quantum defect theory at the R-matrix surface. The initial and final states are represented by configuration-interaction wavefunctions. We calculated total and partial photoionization cross sections in both length and velocity formulations. Photoionization cross sections are dominated by Rydberg series of autoionization resonances converging to Mg+(3p, 4s, 3d and 4p) thresholds. We also calculated the photoelectron angular distribution asymmetry parameter β for the process leaving the ion in the Mg+(3p) state. The present photoionization cross sections are in good agreement with available experiments and previous calculations. There is excellent agreement between length and velocity gauges in our calculation.

Oscillator strengths of allowed and intercombination lines in Si II using non-orthogonal wavefunctions

The importance of valence-shell, core-valence and core-core correlation and interactions between the members of 3s 2 nd 2 D Rydberg series and between the Rydberg series and 3s3p 22 D perturber state in singly ionized silicon has been examined using term-dependent non-orthogonal orbitals in the multiconfiguration Hartree-Fock approach. Large sets of spectroscopic and correlation non-orthogonal functions have been chosen to adequately describe the term dependence of wavefunctions, various correlation corrections and strong interactions in Rydberg series. The relativistic corrections are included through the one-body mass correction, Darwin and spin-orbit operators and two-body spin-other-orbit operator in the Breit-Pauli Hamiltonian. Extensive configuration-interaction wavefunctions have been used in the representation of Si II levels to calculate oscillator strengths and transition probabilities. The accuracy of present oscillator strengths is evaluated by the agreement between the length and velocity formulations combined with the agreement between the calculated and measured transition energies. The present results have been compared with previous calculations, experimental measurements and astronomical observations.

Low energy differential and integral electron-impact cross sections for the 2s22p4 3P → 2p33s 3So excitation in atomic oxygen

Differential and integral cross sectionsxa0(ICSs) for excitation in atomic oxygen have been measured at electron-impact energies of 15, 17.5, 20, 22.5 and 27.5xa0eV. Differential measurements were conducted with a conventional electron energy-loss spectrometer and a microwave discharge source of atomic oxygen. Relative differential cross sections (DCSs) were determined between 15 and 30xa0eV impact energy. With the help of theoretical predictions of the shape of the DCS at large angles, measured results were extrapolated to 180° scattering angle and relative ICSs were deduced. The relative excitation function was normalized to the 30xa0eV impact energy ICS given by Kanik et al (2001 J.xa0Phys.xa0B:xa0At.xa0Mol.xa0Opt.xa0Phys.xa034 2647). Normalization of the ICS values allowed the measured DCSs to be put on the absolute scale. Theoretical calculations of the DCSs were carried out using the R-matrix with the pseudostates approach. A total of 22 spectroscopic bound and autoionizing states, together with 19 pseudostates, were included in the close-coupling expansion. The pseudostates were chosen to simulate continuum target states. Theoretical results, along with other available experimental data, have been compared with the current experimental results.

Strong term dependence of wavefunctions and series perturbations in singly ionized chlorine

The strong interactions between the different 3s23p3nl (l = 0, 2) Rydberg series and the 3s3p5 perturber states in singly ionized chlorine have been investigated using term-dependent non-orthogonal orbitals in the multiconfiguration Hartree–Fock approach. Progressively larger calculations are performed in a systematic manner to evaluate these interactions adequately. The wavefunctions exhibit large correlation corrections. The basis set of functions contains both spectroscopic and correlation orbitals to describe the low-lying Rydberg series members and to account for the higher bound and continuum parts of the series. The mean radii of the correlation orbitals are comparable to the n = 3 and 4 spectroscopic orbitals and thus the correlation corrections are very well represented. There is excellent agreement between the calculated and measured excitation energies and also between the length and velocity formulations of oscillator strength.