Partial cross sections of doubly excited helium below the ionization threshold I7 -: art. no. 042706

Partial photoionization cross sections (PCSs), sigma(n), leading to final ionic states of helium, He+(n), were measured at BESSY II in the region of doubly excited helium up to the ionization threshold I-7 of He+. The experiments were performed with a time-of-flight (TOF) electron spectrometer and high photon resolution, DeltaEcongruent to6 meV. The results of these measurements are a most critical assessment of the decay dynamics of double-excitation resonances and agree well with those of recent eigenchannel R-matrix calculations. They also confirm the propensity rules set up for the autoionization of doubly excitated states. The mirroring behavior in the PCSs predicted recently by Liu and Starace is only partially observed. By discussing the formulas given by these authors in a more general context, the specific behavior of the PCSs of helium with respect to mirroring can be understood. The mirroring compensation properties between the "fractional partial cross sections" gamma(P)=sigma(P)/sigma(T) and gamma(Q)=sigma(Q)/sigma(T), with sigma(T)=sigma(P)+sigma(Q), are introduced and discussed.