Configuration interaction effects in low-energy electron collisions with Fe II

Abstract

Partial wave collision strengths are presented for low-energy electronimpact transitions in Fe II between the 3d 6 4s a 6 D e ground state and the 3d 7 a 4 F e , 3d 6 4s a 4 D e and 3d 7 a 4 P e low-lying excited states. The collision strengths are calculated in LS coupling using a new Fortran 95 R-matrix program including all terms of the 3d 6 4s, 3d 7 , 3d 6 4p, 3d 5 4s 2 and 3d 5 4s4p configurations in the close coupling expansion of the collision wavefunction. Special emphasis is given to the inclusion of configuration interaction (CI) effects both in the target and in the collision wavefunctions. In both cases series of calculations are carried out where additional CI terms are included systematically. It is found that in order to obtain close to converged low-energy partial wave collision strengths two-electron excitations from the 3p shell to the 3d shell as well as pseudo s and d orbitals must be included in the CI expansions. Also resonance effects in low-energy partial wave collision strengths are found to depend sensitively on the representation of the d orbitals in the CI expansion of the collision wavefunction. Finally, CI models for both the target and collision wavefunctions are defined which can be used in proposed calculations to obtain accurate total collision strengths and effective collision strengths for transitions between LS-coupled terms and between fine-structure levels of Fe II.

Electron-impact excitation of Fe II: Effective collision strengths for optically allowed fine-structure transitions

“…For the even parity 3d 6 4s and 3d 7 configuration states, however, the agreement is less satisfactory with typical differences of around 20% for the majority of levels. We also note that considerably better agreement is found for other even parity levels such as the 3d 6 4s 4 D e multiplets (indices [10][11][12][13] where agreement is better than 2%, and by considerably worse agreement, percentage-wise, for the multiplets of 3d 7 ). These ab initio target state energies are adopted in the present collision calculation and no shift was performed to bring the thresholds in line with the observed values as they constituted an incomplete set.…”

“…Subsequent works by Baluja et al [6], Berrington et al [7], Keenan et al [8], Pradhan and Berrington [9], Zhang and Pradhan [10], Bautista and Pradhan [11] and Ramsbottom et al [12][13][14] have increased the sophistication of the theoretical models describing the Fe II target. The most extensive and accurate set of collisional data currently available for the Fe II ion were computed by Ramsbottom et al [15].…”

R-Matrix Scattering Calculations for Iron-Peak Species: Photoionisation of Fe I and Electron-Impact Excitation of Fe II

“…However, the available atomic data for such transitions, obtained from calculations of varying size and complexity, show large discrepancies owing to the difficulty of modelling their open 3d-shell atomic structures. These existing calculations have ranged from small 2-3 term calculations, employing the close-coupling [3] and distorted wave [4] methods, to R-matrix calculations retaining between four and 113 LS terms [5][6][7][8][9] (or 16-142 fine-structure levels for calculations incorporating relativistic effects [6,[10][11][12][13][14]) in the target description, to larger more sophisticated R-matrix calculations of [15,16] including 262 levels in the target description. Currently, there exists little to no agreement amongst these calculations with regards to the effective collision strengths for the forbidden transitions, often exhibiting discrepancies of up to factors of three.…”

Electron-impact excitation of Fe II

“…To explore this convergence and investigate the importance of configuration interaction effects in low energy collisons with Fe ii, a series of calculations were performed by Ramsbottom et al (2002Ramsbottom et al ( , 2004 using the program RMATRXII (Burke et al 1994) and the parallel external R-matrix program PFARM (Sunderland et al 2002) based on FARM (Burke & Noble 1995). In these works additional configuration interaction effects were systematically included both in the target and in the collision wavefunctions.…”