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Abstract

Partial photoionization cross sections (PCSs), σ_n, 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.