Dynamics of the helium atom close to the full fragmentation threshold: Ionization excitation

Abstract

The hyperspherical $\mathcal{R}$-matrix method with semiclassical outgoing waves, designed to provide accurate double-ionization cross sections, is extended to allow for the computation of ionization-excitation data of comparable quality. Accordingly, it appears now as a complete method for treating the correlated dynamics of two-electron atoms, in particular above their full fragmentation threshold. Cross sections ${\ensuremath{\sigma}}_{n}$ and asymmetry parameters ${\ensuremath{\beta}}_{n}$ are obtained for single photoionization of helium with excitation of the residual ion up to as high a level as $n=50$ at $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the double-ionization threshold. These data are extrapolated to infinite values of $n$ in order to check widespread assumptions regarding this limit. Our data are found consistent with the assumed ${n}^{\ensuremath{-}3}$ dependence of the partial ionization cross sections. However, the ${\ensuremath{\beta}}_{\ensuremath{\infty}}=\ensuremath{-}0.636$ obtained still lies far from the $\ensuremath{-}1$ value expected at the double-ionization threshold.