Electron impact excitation of Fe+using the R-matrix method incorporating fine-structure effects

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Abstract
Calculations, using the CRAY X-MP/48 at the SERC Rutherford Appleton Laboratory, are presented for electron impact excitation of Fe+, in which the sixteen fine-structure levels arising from the four lowest states 3d64s 6D, 3d7 4F, 3d64s 4D and 3d7 4P are included. The R-matrix method is used in which terms of the Breit-Pauli Hamiltonian are explicitly included in both the target and the scattering descriptions. Fine-structure collision strengths are thus calculated and comparisons made with the four-state calculations of Nussbaumer and Storey (1980), and with Baluja et al (1986), who used the R-matrix method in LS coupling. The role of the 4p orbital, included in the L2 basis of the Baluja et al calculation, is investigated: it is concluded that their approach is inconsistent unless the corresponding collision channels are also included. It is also inferred that the excitation of the third state, 3d64s 4D, could be strongly affected by coupling to the 3d64p odd parity states, which are not included in any calculation. Excitation rates are therefore presented only for fine-structure transitions between the two lowest states (6D and 4F).