Influence of short-range interference on ionization threshold law

Electron-impact ionization of the collinear Z=1/4 model atom is investigated in order to examine a nonclassical behavior of the ionization cross sections. Slightly above the ionization threshold, nb initio calculations reveal a distinctive dip due to short-range dynamics. The dip is a strongly energy-dependent feature in the usually smooth and structureless ionization cross section and is foreign to treatments based on classical, as well as semiclassical mechanics. The Z=1/4 model thus serves as a counterexample to the standard Wannier treatment of near-threshold ionization, The hyperspherical hidden-crossing theory is applied to identify the origin of the dip.