PARTIAL PHOTOIONIZATION CROSS-SECTIONS AND PHOTOELECTRON ANGULAR-DISTRIBUTIONS FOR DOUBLE EXCITATIONS UP TO THE N=5 THRESHOLD IN HELIUM

Partial photoionization cross sections sigma(n) and photoelectron angu lar distributions beta(n) were measured for all possible final ionic s tates He+(n) in the region of the double excitations N(K,T)(A) up to t he N=5 threshold. At a photon energy bandpass of 12 meV below the thre sholds N=3,4, and 5, this level of differentiation offers the most cri tical assessment of the dynamics of the two-electron excitations to da te. The experimental data are very well described by the most advanced theoretical calculations. Weaker double-excitation series with K=N-4 are clearly visible in the beta(n) data, and even previously unobserve d extremely weak series members with A=-1 can be discerned, showing th e high sensitivity of the angular resolved measurements. The shapes of the resonance-induced variations of sigma(n) or beta(n) in the double excitations below a given threshold N change radically depending on t he final ionic state n but display striking similarities when comparin g the satellite states with n=N-1 and n=N-2 below each threshold N. Th ese systematic patterns may indicate a general rule for the underlying two-electron dynamics.