Partial Photoionization Cross Section of Collinear eZe Helium: Numerical Confirmation of Semiclassical Predictions

Based on the semiclassical theory of chaotic scattering, Tanner et al. [J. Phys. B 40, F157 (2007)] proposed the fluctuation in the partial photoionization cross section of helium below the double-ionization threshold would show the same characteristics as in the total cross section, predicting that the Fourier spectrum of the fluctuation reveals peaks at the classical actions of closed triple collision orbits and the amplitude of the fluctuation decreases algebraically as the energy approaches the double-ionization threshold. In that paper, however, the predictions were not clearly confirmed due to the lack of experimental data with sufficient accuracy. So instead, we calculate the partial photoionization cross sections of collinear eZe helium for the energy range from the single-ionization threshold $I_{20}$ to $I_{32}$ in order to numerically confirm the predictions. Analysis of the fluctuation in the partial cross section shows that the predictions are indeed valid. Our findings mean that the fluctuation in the partial photoionization cross section can be described by classical triple collision orbits in the semiclassical limit. Thus it explains in a natural way the mirroring and mimicking structures observed in cross section signals for different ionization channels.