Doubly excited states of helium observed in N- and l-specific partial photoionization cross-sections using lifetime-resolved fluorescence spectroscopy

We have used a lifetime-resolved fluorescence technique to obtain the first measurements of partial photoionization cross-sections of ground state neutral helium with both N- and l-specific final ion state separation. Structure is observed in the He 4lnl' doubly excited state regions and the resulting resonance profiles and energy positions qualitatively compared to previously reported theoretical calculations. Due to the sensitive nature of the N- and l-specific partial cross-sections, states from the (4,−2)n series are observed for the first time.