MEASUREMENTS IN THE PERPENDICULAR PLANE OF ANGULAR-CORRELATIONS IN NEAR-THRESHOLD ELECTRON-IMPACT IONIZATION OF HELIUM

Angular correlation measurements have been made of helium (e, 2e) scattering in the perpendicular plane, defined as the plane normal to the direction of the incident electron beam, at excess energies of 1, 2, 4 and 6 eV. The simultaneous analysis of these measurements with the coplanar measurements of Selles et al gives the value 74 +/- 7-degrees for the Wannier parameter-theta-0 that governs the widths of the angular correlation functions. It is also concluded that the upper limit of the excess energy for which the Wannier model is valid in helium is approximately 1.5 eV for coplanar scattering but appears to be higher for scattering in the perpendicular plane.