Single photon double ionization of the He-like Li$^+$ ion

The success of the R-matrix plus pseudo-state (RMPS) method to model single photoionization, is exploited and extended to calculate the single photon double ionization cross-section for the He-like ion, Li$^+$, from both the ground state and the n=2 excited levels. Comparisons of the present R-matrix plus pseudo-state (RMPS) results are made with other theoretical methods such as time-dependent close-coupling (TDCC), the convergent close-coupling (CCC) and the B-spline approach. Suitable agreement between the various theoretical approaches are achieved but some differences occur which are high-lighted and discussed. The maxima in the respective cross sections are extremely small, being in the region of 2Kb for the ground state and 1Kb and 6 Kb, for the n=2 meta stable states which makes their experimental determination very challenging.