Electron impact double ionization of multiply charged iron ions

Using the crossed-beams technique, cross sections for electron impact doubl e ionization of Feq+ ions in charge states q = 1-6 (except 2) are measured in the energy range from the respective ionization thresholds up to 1 keV, partly up to 5 keV. The data for Fe4+ Fe5+ and Fe6+ show significant contri butions from inner-shell processes. Using a high-resolution energy-scan tec hnique, we study narrow resonances in the cross sections for Fe4+ and Fe5+. Besides contributions from excitation-autoionization processes, a remarkab le number of structures arising from resonant-excitation triple-autoionizat ion processes are observed. The results are compared with the scaling laws of Fisher et al and the semiempirical formula published by Shevelko and Taw ara, respectively.