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

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 3d 6 4s 6 D, 3d 7 4 F, 3d 6 4s 4 D and 3d 7 4 P 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 L 2 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, 3d 6 4s 4 D, could be strongly affected by coupling to the 3d 6 4p 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 ( 6 D and 4 F).