ABSOLUTE FULLY RESOLVED (e,3e) CROSS SECTIONS
FOR THE DOUBLE IONIZATION OF HELIUM


A. Lahmam-Bennani1$,  A. Duguet1, I. Taouil1 L. Avaldi2 and J. Berakdar3


1Laboratoire des Collisions Atomiques et Moléculaires (UMR 8625), Bât 351,
Université Paris XI, F-91405 Orsay Cedex, FRANCE
2IMAI del CNR, Area della Ricerca, CP 10, 00016 Monterotondo Scalo, ITALY
3Max-Planck Institut für Mikrostrukturphysik, Weinberg 2, 06120 Halle, GERMANY


        We have measured for the first time full sets of coplanar (e,3e)
angular distributions for double ionization of He, at an impact energy of
~5 and ~1 keV. Ejected electrons are detected with equal energies, 10 and 4
eV. Comparison is made with analog (g,2e) results, and with calculations
performed using a four-body final-state wavefunction for the three
electrons moving in the field of He2+. Deviations from the dipolar limit
are pointed out, and the origin of dips and peaks in the spectra is
analyzed. General trends for other targets are proposed in terms of double
ionization mechanisms.