Observation of doubly excited states in negative ions using resonance ionization spectroscopy

Negative ions are especially sensitive to electron correlation since in these loosely bound systems the electron-electron interaction can become comparable to or dominate the electron-core interaction. This statement is particularly true in the case of double excitation where two electrons are simultaneously excited to large distances from the residual core. In this paper it is demonstrated that RIS is a powerful tool for investigations of doubly excited states of negative ions under the simultaneous condition of high resolution and sensitivity. A proof-of-principle experiment is presented, where doubly excited states of Li- situated just below the Li(4p) are investigated. Such autodetaching states manifest themselves as resonances in the photodetachment cross section. We have measured the photon energy dependence of the partial cross section for leaving the residual Li atom in the 3s state. Fano profiles were fitted to the individual resonances in order to extract the energies and widths of the doubly excited states.