High-resolution photoion yield measurements of 'hollow' atomic lithium

Using monochromatized extreme-UV radiation from the undulator beamline BW3 at HASYLAB we have measured at high resolution (E/ Delta E approximately 7000) the Li + photoion yield spectrum at excitation energies corresponding to transitions to double K-shell vacancy states. A number of such resonances including single-photon-three-electron excitations, e.g. 1s 2 2s( 2 S) to 2p 3 ( 2 p), have been observed for the first time. The spectrum is tentatively classified with the aid of Hartree-Fock calculations.