Evidence of triple collision dynamics in partial photo-ionization cross sections of helium

Experimental results on partial photo-ionization cross sections of helium are analysed in the light of recent advances in the semiclassical theory of two-electron atoms. Byun et al (2007 Phys. Rev. Lett. 98 113001) predict that the total photo-ionization cross section below the double-ionization threshold can, semiclassically, be described in terms of contributions associated with classical orbits starting and ending in the triple collision. The necessary modifications of the semiclassical theory for partial cross sections are developed here. It is argued that partial cross sections are also dominated by the triple collision dynamics. The expected semiclassical contributions can be identified in the Fourier transformation of the experimental data. This clearly demonstrates for the first time the validity of the basic assumptions made by Byun et al. Our findings explain furthermore in a natural way the self-similar structures observed in cross section signals for different channel numbers.