NEAR-THRESHOLD TDCS FOR PHOTO-DOUBLE IONIZATION OF HELIUM

TDCS for photo-double ionization of helium have been measured in a Pho toElectron-PhotoElectron COincidence (PEPECO) experiment. The TDCS hav e been obtained for the first time at very low excess energies E, 0.6 eV < E < 2 eV, for both equal and unequal energy sharing between the t wo outgoing electrons. The measured data are compared with the predict ions of the Wannier model, and also with recent, non-Wannier, ab initi o calculations. In addition, also for the first time, the relative mag nitudes of the various TDCS measured have been determined in this exce ss energy region, suggesting a departure from the predictions of the W annier model at the largest excess energy studied, E = 2 eV.