Partial photoionization cross sections and angular distributions for double excitation of helium up to the N=13 threshold -: art. no. 243003

Partial photoionization cross sections sigma(N)(E-gamma) and photoelectron angular distributions beta(N)(E-gamma) were measured for the final ionic states He+(N > 4) in the region between the N=8 and N=13 thresholds (E-gamma > 78.155 eV) using the cold target recoil ion momentum spectroscopy technique (COLTRIMS). Comparison of the experimental data with two independent sets of theoretical predictions reveals disagreement for the branching ratios to the various He-N(+) states. The angular distributions just below the double ionization threshold suggest an excitation process for highly excited N states similar to the Wannier mechanism for double ionization.