Total photoionization cross section of planar helium: scaling laws and collision orbits

Abstract

The total photoionization cross section of planar helium has been calculated up to the single ionization threshold I-22 of triple P states. The cross section shows chaotic fluctuations as the energy E approaches the double ionization threshold E = 0. By analyzing the fluctuating part of the cross section, we show that its amplitude decreases as vertical bar E vertical bar(mu) for E -> 0(-) as predicted in Byun et al (2007 Phys. Rev. Lett. 98 113001). The Fourier transform of the fluctuating part reveals peaks at the classical actions of closed triple collision orbits. Furthermore, the relative height of the peaks is consistent with the semiclassical predictions. Our findings underline that the fluctuating part of the photoionization cross section can be described by classical triple collision orbits in the semiclassical limit. These orbits all lie in the collinear eZe subspace, demonstrating that the fluctuations are dominated by the dynamics of this low dimensional phase space.