Partial photoionization cross sections and photoelectron angular distributions for double excitations up to the N=5 threshold in helium.

Abstract

Partial photoionization cross sections ${\mathrm{\ensuremath{\sigma}}}_{\mathit{n}}$ and photoelectron angular distributions ${\mathrm{\ensuremath{\beta}}}_{\mathit{n}}$ were measured for all possible final ionic states ${\mathrm{He}}^{+}$(n) in the region of the double excitations N(K,T${)}^{\mathit{A}}$ up to the N=5 threshold. At a photon energy bandpass of 12 meV below the thresholds N=3,4, and 5, this level of differentiation offers the most critical assessment of the dynamics of the two-electron excitations to date. 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 ${\mathrm{\ensuremath{\beta}}}_{\mathit{n}}$ data, and even previously unobserved extremely weak series members with A=-1 can be discerned, showing the high sensitivity of the angular resolved measurements. The shapes of the resonance-induced variations of ${\mathrm{\ensuremath{\sigma}}}_{\mathit{n}}$ or ${\mathrm{\ensuremath{\beta}}}_{\mathit{n}}$ in the double excitations below a given threshold N change radically depending on the final ionic state n but display striking similarities when comparing the satellite states with n=N-1 and n=N-2 below each threshold N. These systematic patterns may indicate a general rule for the underlying two-electron dynamics. \textcopyright{} 1996 The American Physical Society.