Systematics of energies of doubly excited states of heliumlike ions

The dependence of the energy levels of doubly excited states of heliumlike ions on the nuclear charge Z is studied. By expressing the energy levels of doubly excited states for intrashell states in the form of a double-Rydberg formula and for intershell states using the quantum-defect method, we expand the quantum defect in each formula in powers of 1/Z. Using known energy levels for the low-Z ions, we extract parameters of the quantum defect which allow the calculation of accurate energy levels of other ions along the helium isoelectronic sequence.