Relativistic and electron-correlation effects in electron-impact excitation of Fe2+

A study of the relative importance of the relativistic and the electron-correlation effects in electron-impact excitation of Fe2+ is carried out in the close-coupling approximation using the non-relativistic and the Breit-Pauli R matrix methods. Results from several sets of calculations with different eigenfunction expansions show that the relativistic effects for Fe2+ are relatively unimportant compared to electron correlation. The n-state close-coupling calculations with nLS terms (nCC) include 5cc, 7cc, and 83cc expansions, corresponding to 17, 23, and 219 fine-structure levels respectively, dominated by electronic configurations 3d6, 3d54s and 3d64p. The relativistic collision strengths are shown to be affected significantly by correlation between some of the levels in the intermediate-coupling expansion, and the results differ significantly from earlier work. Detailed autoionization resonance structures also play an important role. The present study provides the basis for large-scale collisional calculations for Fe2+ for applications in spectral diagnostics of a variety of astrophysical sources. (Author)