Energy Correlation of the Three Electrons Emitted during the Triple Photoionization of Ar

We report on an experimental investigation of energy correlation among three electrons emitted in valence triple photoionization (TPI) of Ar. The energy correlations reveal a predominant contribution from sequential TPI processes involving intermediate Ar2+ formation, which suggests that such indirect contributions should be included in the formulation of the threshold law for TPI cross sections. The differential cross section for direct TPI at about 150 eV above threshold producing one slow electron with a few eV kinetic energy is found to have a deeply hollow U-shaped profile in energy sharing between the other two electrons.