ONE-ELECTRON AND 2-ELECTRON EXCITATIONS OF HELIUM IN THE S-WAVE MODEL

The energies of the bound states and energies and widths of autoionizi ng resonant states of helium are calculated within the s-wave model, w here the individual orbital angular momenta of both electrons are zero . The energies of the bound 1sns states differ from the corresponding energies in real helium only via a small n-independent shift Delta mu in the quantum defects, which amounts to Delta mu=0.011 for singlet st ates and Delta mu=0.004 for triplet states. The quantum defects of mor e than 50 bound and resonant states with singlet or triplet symmetry a re reproduced by an empirical four-parameter formula to within an rms deviation of less than 0.016. The normalized widths of the autoionizin g Nsns resonant states increase with the smaller quantum number N, and the widths of the singlet states tend to become larger than the separ ation of successive resonances in the Rydberg series for N approximate to 8. Effects of interference of Rydberg series can be described in t he framework of multichannel quantum-defect theory. In the 7sns single t series of resonances, interference due to the 8s8s perturber inhibit s autoionization by more than three powers of IO. Semiclassical quanti zation based on unstable periodic orbits reproduces the energies of st ates with equal or similar quantum numbers rather well in a standard a pplication of the cycle-expansion technique and very well in an applic ation using only three nonretracing periodic orbits.