Determination of the 2s-2p excitation energy of lithiumlike scandium using dielectronic recombination

High-resolution spectroscopy of doubly excited states produced by dielectronic recombination (DR) of lithiumlike Sc18+ ions was performed by employing the electron-ion merged-beam technique at the heavy-ion storage ring TSR. The experimental procedure for measuring DR resonances with high precision is thoroughly described with an emphasis on the uncertainties of the experimental energy scale. Absolute measurements of recombination rate coefficients were carried out over the center-of-mass energy range 0-50 eV that comprises all DR resonances associated with 2s(1/2)->-> 2p(1/2,3/2) excitations. At relative energies below 300 meV resonances due to DR via Sc17+ (1s(2) 2p(3/2) 10l(j)) intermediate states were found. Their positions could be measured with an uncertainty of only +/- 1.8 meV. The results are compared with theoretical calculations within the framework of relativistic many-body perturbation theory. By combining the precision of the experimental and theoretical results we derive a value for the 2s(1/2)->-> 2p(3/2) excitation energy, 44.3107(19) eV, which is by more than an order of magnitude more accurate than the hitherto most precise value obtained from optical spectroscopy.