New Window Resonances in the Potassium 3s Photoabsorption Spectrum
  The photoion spectrum of atomic potassium was measured over the 3s -> np excitation region with the photoion time-of-flight method and monochromatized synchrotron radiation. An unusual spectrum with paired windows structure was found instead of a simple regular Rydberg series. Such subsidiary windows have not been observed in the 3s -> np resonances of Ar, which has a closed outer shell. Based on Dirac-Fock calculations, the dual window structure at 36.7 eV and at 37.4 eV was assigned to the 3s^{-1}3p^{6}4s4p resonance. The line shape can be fitted by Fano's formula and the Fano parameters were obtained. 
M. Koide, F. Koike, R. Wehlitz, M.-T. Huang, T. Nagata, J. C. Levin,   S. Fritzsche, B. D. DePaola, S. Ohtani, and Y. Azuma