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

The hyperspherical 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 sigma(n) and asymmetry parameters 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 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(-3) dependence of the partial ionization cross sections. However, the beta(infinity)=-0.636 obtained still lies far from the -1 value expected at the double-ionization threshold.