Double shake-off model for the triple photoionization of beryllium

We propose a model for the triple photoionization of Be in which a core 1s electron absorbs the photon γ + 1s²2s² Be → ε₁p + 1s2s² Be⁺ and the valence 2s² electrons are shaken off into continuum due to the sudden change of the core potential. We decompose the double shake-off amplitude into a single shake-off 2s² → nsεs and a subsequent electron impact ionization of the doubly charged Be²⁺ ion εs + 1sns Be²⁺ → ε₂l + ε₃l + 1s Be³⁺. The latter process is described by the T-matrix of inelastic electron scattering on the 'semi-hollow' 1sns Be²⁺ ion in the monopole singlet channel. The convergent close-coupling method is used to evaluate the T-matrix.