The search for oscillations in the near-threshold photo-double ionization cross section of helium

The photo-double ionization cross section for the helium atom is calculated in the near-threshold region by direct solution of the time-dependent Schroedinger equation. Full close-coupling results for the 1s21S ground state are found to be in excellent agreement with experimental measurements. The calculations confirm the validity of the Wannier power law from 0.1 eV to about 1.7 eV excess energy and find no oscillations in the threshold cross section beyond numerical uncertainty. Further time-dependent calculations are made in a simpler s-wave counterlinear model for both the 1s21S ground and 1s2s 1S excited states. Although numerical uncertainties are significantly reduced in the helium model calculations, again no oscillations in the threshold cross sections are found beyond the remaining numerical uncertainty. (letter to the editor)