Relativistic and resonance effects in electron impact excitation ofFe5+

Abstract

Relativistic and resonance effects in electron impact excitation of Fe5+ are investigated using four sets of close-coupling calculations with the R-matrix method. The different eigenfunction expansions for the N-electron target ion consist of eight LS terms, 19 fine structure levels, dominated by the ground configuration 3d3, and 34 LS terms (80 fine structure levels) dominated by additional configurations 3d24s and 3d24p. Relativistic effects are considered in two ways: a full intermediate coupling 19-level Breit-Pauli (BP) approximation, and using the term-coupling coefficients for the target levels. Unlike the lower ionization stages of Fe, it is necessary to consider explicitly the fine structure in the target in order to delineate precisely the positions of autoionizing resonances and detailed resonance structures. Otherwise, the relativistic effects are small, as indicated by the Maxwellian averaged collision strengths from the different approximations. It is found that the presence of particular bound-channel resonances, corresponding to (N + 1)-electron correlation functions in the close-coupling expansion for the (e- + ion) system, has a marked effect on the near-threshold cross sections. General implications of this work are discussed. As these resonances represent real physical features they should not be treated as pseudo-resonances. It is shown that many resonant features are well reproduced by the chosen (N + 1)-electron basis set of correlation functions in the smaller eight-term LS and 19-level BP calculations, as verified by comparison with the 34-term calculation. The present collision strengths, with large resonance structures, differ considerably from earlier calculations without resonances.