MULTICOINCIDENCE MEASUREMENTS OF DOUBLE PHOTOIONIZATION IN HELIUM

A new toroidal analyser has been used to study the double photoionizat ion of helium, for energies down to 4 eV above threshold. The two elec trons are detected with equal energies, and coincidences between them for various angles of emission can be obtained simultaneously. The pla ne of detection contains the photon beam, in contrast with recent expe riments performed in the plane perpendicular to the photon beam. The a ngular correlation function is extracted directly from the present mea surements, which are shown to be insensitive to the Stokes parameters of the photon beam. The width of that function is not observed to vary significantly from 4 eV to 17.6 eV above threshold.