H E Saraph

Atomic data for opacity calculations. IX. The lithium isoelectronic sequence

Bound-state energies, oscillator strengths and photoionisation cross sections have been calculated for members of the lithium isoelectronic sequence with nuclear charge Z in the range 3<or=Z<or=10. Two independent approaches to the problem give virtually the same results. Detailed comparisons with experiment and other theoretical results indicate that the present data are of high accuracy.

Oscillator strengths for dipole transitions in neutral oxygen

Accurate bound-state wavefunctions obtained by the close-coupling method in the frozen-cores approximation are used to calculate oscillator strengths for neutral oxygen. Results include transition probabilities for all terms of O I (excited and ground states) obtained by adding an s, p or d electron to the ground-configuration terms of O II. The authors consider Rydberg series up to n=6. Many of the transitions are parentage forbidden and configuration mixing provides the driving mechanism. Results agree with experiment to within 10%. Strong intercombination lines are observed in oxygen; the effects of deviations from LS coupling on the allowed transitions presented are discussed by using the unpublished results from an independent investigation by Zeippen (1977). Calculations include the continuum region, and the photoionisation cross section for oxygen is obtained. These results are in very satisfactory agreement with close-coupling calculations by Taylor and Burke 1976.

Calculation of electron scattering by neutral oxygen, using an accurate representation for the target

A multi-configuration expansion is used to represent the three lowest terms of the oxygen atom. Total cross sections for electron scattering by this target are in good agreement with experiments by Sunshine et al (1965).

A comparison of close-coupling calculations using UCL and QUB codes

Abstract Close-coupling (CC) calculations are made, for the (C2+ + e) system, using two completely independent sets of codes: (i) University College London (UCL) codes in which the CC equations are solved using IMPACT; (ii) Queens University of Belfast (QUB) codes, in which the equations are solved using RMATRX, together with new fast outer-region codes. Results are obtained for energies En of C+ bound states, C+ line strengths S and (C2++e) collison strengths ω. Result for energie are expressed in terms of effective quantum members, vn, and it is found that the two sets of codes gives values of vn agreeing to within about 0.001. Results for S agree to within better than 1% except for cases for which there is severe cancellation in the dipole matrix elements. Results for ω agree to within about 0.1% except for energies in the immediate vicinities of narrow resonances. There is good agreement for positions and widths of resonances.

Resonances in cross sections for excitation of forbidden lines in O2

In previous work, Saraph, Seaton and Shemming have calculated cross sections for electron impact excitation of the O2+ ground configuration terms, in the approximation of neglecting collisional coupling to configurations other than 2s22p2. In the present work it is shown that collisional coupling with 2s2p3 produces resonances of the type 2s2p33s in the near-threshold cross sections. Calculations are made using semi-empirical methods. Radial functions are calculated using a statistical-model potential, and the binding energy of the 3s electron in the state 2s2p3(5S)3s 4S is calculated allowing for configuration interaction with 2s2p3(3S)3s 4S and 2s22p3 4S. A parameter in the potential is then varied so as to obtain agreement with the observed binding energy for this state. Similar calculations are then made for 2s2p3(3D)3s 2D allowing for configuration interaction with 2s2p3(1D)3s 2D and 2s22p3 2D, and for 2s2p3(3P)3s 2P allowing for interaction with 2s2p3(1P)3s2P and 2s22p3 2P. It is found that the 3s 2D state lies just above the threshold for excitation of 2s22p2 1D, and that the 3s 2P state lies just above the threshold for excitation of 2s22p2 1S. A generalized reactance matrix is calculated using a variational expression, and the 2s2p33s functions of the present work and the 2s22p2kp functions obtained by Saraph, Seaton and Shemming. Resonance structures are calculated using methods of quantum defect theory. It is found that the resonances produce important modifications in the cross sections at near-threshold energies of interest for astrophysical applications.

O IV: bound states, oscillator strengths and photoionisation cross sections

Close-coupling calculations were performed to obtain wavefunctions for the O4++e scattering problem with incident energies between -5.667 and +0.95 Ryd. This energy range covers bound states and quasi-bound states of the O3+ ion from 2s22p2P0 to 2p2(3P)4d2D. The six terms arising from configurations 2s2, 2s2p, and 2p2 of the target O4+ were included in the calculation. The authors predict a number of high members of Rydberg series and doubly excited states hitherto not observed. Oscillator strengths for optically allowed transitions are given between most terms calculated and photoionisation cross sections for fifteen terms of O3+. The background behaviour of parentage allowed photoionisation cross sections can be simply calculated using a formula and program developed by Peach. It is shown that high bound states 2s2nl can contribute significantly to the population of states of the excited configuration 2s2p.

Use of frozen-cores wavefunctions for the calculation of oscillator strengths in Mg+ and Mg

The integro-differential equations for electron-ion scattering processes are solved for bound-state energies of the compound system, a method called frozen-cores approximation when introduced by Wilson and Seaton (1972). The solutions are used to calculate oscillator strengths. The core states chosen in the present paper are such that the results should be comparable with the recent multiconfiguration Hartree-Fock calculations for Mg by Froese Fischer (1975).

Excitation of Ca+ by electron impact: polarization of line radiation and transitions between fine-structure levels

R matrices calculated by Burke and Moores (1968) for electron impact excitation of the 3d 2D and 4p 2P levels in Ca+ are used. Collision strengths for excitation of fine-structure levels are obtained at energies below and above the threshold for excitation of 4p 2P. These collision strengths are of interest in astrophysics. The polarization of radiation emitted from 2P is related to the cross sections for excitation of the 2P components. Calculated values for these cross sections and for the polarization of the 2P-2S multiplet are given for a range of excitation energies.

On the convergence of close-coupling expansions for electron impact excitation N+

Collision strengths for N+ 2s22p2 3P-1D and 3P-1S have been calculated allowing for configuration interaction between 2s22p2 and 2p4 and with inclusion of 2s2p3 states in the close coupling expansion. Use of the same approximations for the calculation of bound states in neutral N indicates that the collision strengths should be correct to within about 10%. Inclusion of the 2s2p3 states does not give a large change in the near-threshold collision strengths; this result is in disagreement with that obtained by Ormonde et al (1971) and Ormonde et al (1973).

Electron impact transitions between fine structure levels in ions with configurations 1s22s22p3

Collision strengths are calculated for transitions between the levels 4S3/2, 2D3/2, 2D5/2, 2P1/2 and 2P3/2 in the ions O+, F2+, Ne3+ and Na4+, with configurations 1s22s22p3. Below the 2P threshold the collision strengths are averaged over resonances in series converging onto 2P.