Interference effects in the photorecombination of argonlike Sc3+ ions: Storage-ring experiment and theory

Absolute total electron-ion recombination rate coefficients of argonlike Sc3+(3s23p6) ions have been measured for relative energies between electrons and ions ranging from 0 to 45 eV. This energy range comprises all dielectronic recombination resonances attached to 3p→3d and 3p→4s excitations. A broad resonance with an experimental width of 0.89±0.07 eV due to the 3p53d22F intermediate state is found at 12.31±0.03 eV with a small experimental evidence for an asymmetric line shape. From R-matrix and perturbative calculations we infer that the asymmetric line shape may not only be due to quantum-mechanical interference between direct and resonant recombination channels as predicted by Gorczyca et al. [Phys. Rev. A 56, 4742 (1997)], but may be partly also due to the interaction with an adjacent overlapping dielectronic recombination resonance of the same symmetry. The overall agreement between theory and experiment is poor. Differences between our experimental and our theoretical resonance positions are as large as 1.4 eV. This illustrates the difficulty to accurately describe the structure of an atomic system with an open 3d shell with state-of-the-art theoretical methods. Furthermore, we find that a relativistic theoretical treatment of the system under study is mandatory since the existence of experimentally observed strong 3p53d2 2D and 3p53d4s 2D resonances can only be explained when calculations beyond LS coupling are carried out