Partial photoionization cross sections of doubly excited helium below the ionization thresholds I₈ and I₉

Partial photoionization cross sections (PCSs), σn, leading to final states of singly ionized helium, He⁺(n), were measured in the region of doubly excited helium below the ionization thresholds I8 and I9. The experiments were performed at BESSY II at high photon resolution, ΔE=~6 meV, using a time-of-flight electron spectrometer. A comparison with recent eigenchannel R-matrix calculations reveals good agreement. The results of these measurements underline previous studies on quantum chaos in helium, which were mainly based on theoretical results. They also allow a critical assessment of the theoretical methods that produce the data used for a statistical analysis of double-excitation states with respect to quantum chaos, which is expected to occur very close to the double-ionization threshold. PCSs provide additional information to that derivable from total cross sections (TCSs). In the present PCS spectra, the resonance 8, 410 of the secondary series, which could not be resolved in the TCS, is clearly observed