A HYPERSPHERICAL CLOSE-COUPLING CALCULATION OF PHOTOIONIZATION FROM THE HE ATOM, LI+ AND C4+ IONS .1. BELOW THE N = 2 THRESHOLD

We give a detailed account of the formulation of the recently develope d hyperspherical close-coupling (HSCC) method. Using this method, we h ave made calculations of photoionization cross sections for the He ato m and He-like ions, Li+ and C4+ below the N = 2 (the inner electron's principal quantum number) threshold. From analysing the photoionizatio n cross section spectra and resonance structures of the two-electron s ystems with different nuclear charge, the effect of the interplay betw een the electron-nucleus and electron-electron interactions on doubly excited states is studied.