Double Photoionization of Lithium Revisited

In a previous paper\footnote{R. Wehlitz, J.B. Bluett , and S.B. Whitfield, Phys. Rev. Lett. {\bf89}, 093002 (2002)} we believed to have seen oscillations in the double-photoionization cross section of lithium. However, a recent investigation revealed that resonances at twice the photon energy (compared to the near-threshold energy region) are more pronounced than had been expected\footnote{R. Wehlitz and P.N. Jurani\'c, Phys. Rev. A {\bf 74}, 042721 (2006)}. This prompted us to revisit the near-threshold region of the lithium double-to-single photoionization ratio. Using a slightly higher energy resolution in a new experiment on the same beamline, we could identify resonances in that ratio due to second-order light. While the second-order light contribution is small, so is the double-photoionization cross section in first-order light near threshold. The ``resonances'' observed near threshold match the inner-shell resonances at twice the photon energy fairly well and can indeed explain the previously seen ``oscillations''.