Analysis of the fine structure of Sn$^{11+...14+}$ ions by optical   spectroscopy in an electron beam ion trap
  We experimentally re-evaluate the fine structure of Sn$^{11+...14+}$ ions. These ions are essential in bright extreme-ultraviolet (EUV) plasma-light sources for next-generation nanolithography, but their complex electronic structure is an open challenge for both theory and experiment. We combine optical spectroscopy of magnetic dipole $M1$ transitions, in a wavelength range covering 260\,nm to 780\,nm, with charge-state selective ionization in an electron beam ion trap. Our measurements confirm the predictive power of \emph{ab initio} calculations based on Fock space coupled cluster theory. We validate our line identification using semi-empirical Cowan calculations with adjustable wavefunction parameters. Available Ritz combinations further strengthen our analysis. Comparison with previous work suggests that line identifications in the EUV need to be revisited. 
A. Windberger (1 and 2), F. Torretti (1 and 3), A. Borschevsky (4), A.   Ryabtsev (5 and 6), S. Dobrodey (2), H. Bekker (2),...
physics.atom-ph