Frozen-core model of the double photoionization of beryllium

We calculate the ionization-excitation and double ionization cross sections of the valence 2s2 shell of beryllium. Our model combines a multiconfiguration Hartree-Fock expansion of the beryllium atom ground state and a momentum space close-coupling expansion of the final ionized state. A near-complete set of negative and positive energy pseudostates is employed to represent various singly and doubly ionized channels. The role of the frozen 1s2 core is elucidated by comparing the beryllium single and double photoionization cross sections with those of the 'hollow' helium 2s2 atom in which the 2s orbital is made orthogonal to the vacant 1s orbital. The angular correlation in the two-electron continuum is studied by calculating the triply differential cross section of Be at equal energy sharing between the photoelectrons