Electronic correlation effects in a model of endohedral Mg (Mg at C60)

We have studied theoretically the atomic structure and ground-state photoionization cross-section in a model of endohedral Mg (Mg at C60) with particular emphasis on doubly excited bound and autoionizing (AI) states. We modelled Mg at C60 as a system of two active electrons outside a frozen, doubly charged ionic core, confined to a spherically symmetric potential well of finite depth. We employed a suitable B-spline basis set within a CI approach to describe the two-electron states of the confined system. In addition, we have calculated the ground-state photoionization cross-section in a limited energy range above the first ionization threshold. We found that doubly excited perturbers appear in each regular singly excited Rydberg series of bound states. They are composed of two-electron configurations built with those one-electron orbitals mostly affected by the confining potential. Moreover, the low-lying part of the AI spectrum for each symmetry analysed is perturbed by states novel to the confined system, whose configuration expansion is again dominated by the one-electron orbitals mostly affected by the confining potential and whose AI widths vary over a wide range of values. Finally, the ground-state photoionization cross-section in a limited energy range (a few eV) above the first ionization threshold is structured by the AI resonances in a way markedly different from the corresponding Mg atom cross-section