Above-threshold double-ionization spectroscopy of argon - art. no. 053405

We have measured the above-threshold ionization (ATI) electron spectrum fro m nonsequential double ionization of Ar, and compare our results to the sin gle-ionization ATI spectrum. A variation on the coincidence-tagging techniq ue enables the subtraction of contamination associated with single ionizati on, revealing an excess of higher energy electrons with measured kinetic en ergies 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.