Resonant Electron Impact Excitation of 3d Levels in Fe14+ and Fe15+

We present laboratory spectra of the 3p-3d transitions in Fe¹⁴⁺ and Fe¹⁵⁺ excited with a mono-energetic electron beam. In the energy-dependent spectra obtained by sweeping the electron energy, resonant excitation is confirmed as an intensity enhancement at specific electron energies. The experimental results are compared with theoretical cross sections calculated based on fully relativistic wave functions and the distorted wave approximation. Comparisons between the experimental and theoretical results show good agreement for the resonance strength. A significant discrepancy is, however, found for the non-resonant cross section in Fe¹⁴⁺. This discrepancy is considered to be the fundamental cause of the previously reported inconsistency of the model with the observed intensity ratio between the {}³{P}₂{--}{}³{D}₃ and {}¹{P}₁{--}{}¹{D}₂ transitions.