Nuclear excitation by electron capture in excited ions*

Since its recent observation in the ⁹³Mo isomer depletion, nuclear excitation by electron capture (NEEC) is the object of a live debate: the large measured excitation probability P_exc differs by nine orders of magnitude from the state-of-the-art theoretical predictions. A new study slightly increased this theoretical limit, accounting for the momentum distribution of the target electrons. Authors showed that the depletion probability increases by several orders of magnitude in case of the L-shell. However, these L-channels do not contribute significantly to the P_exc. So far, the evaluation of the NEEC cross section has been carried out widely using the assumption that the ion is in its electronic ground state prior to the capture (GSA), prohibiting the capture in the innermost-shells as soon as the atomic orbitals fill up. For ⁷³Ge this means that NEEC into K-shell cannot occur: the energy released through a K-capture exceeds the nuclear transition energy. By lifting this restriction and considering NEEC in excited ions, we show for ⁷³Ge that many more capture channels emerge. These excited electronic configurations make K-capture now possible, through new channels with resonance strengths larger than any other channel obtained under GSA.