We have developed a two-electron outer region for use within $R$-matrix theory to describe double ionization processes. The capability of this method is demonstrated for single-photon double ionization of He in the photon energy region between 80 and 180 eV. The cross sections are in agreement with established data. The extended $R$-matrix with time dependence method also provides information on higher-order processes, as demonstrated by the identification of signatures for sequential double ionization processes involving an intermediate ${\mathrm{He}}^{+}$ state with $n=2$.

• Received 2 April 2015

DOI:https://doi.org/10.1103/PhysRevA.92.022504