Above-threshold double-ionization spectroscopy of argon

We have measured the above-threshold ionization (ATI) electron spectrum from nonsequential double ionization of Ar, and compare our results to the single-ionization ATI spectrum. A variation on the coincidence-tagging technique enables the subtraction of contamination associated with single ionization, revealing an excess of higher energy electrons with measured kinetic energies up to approximately 2U(p). We also find an excess of very low energy photoelectrons, not previously observed. Implications for the rescattering model are discussed.