Electron collisions with metastable helium

Measurements and theoretical studies of electron scattering from the metastable 21,3S excited states of helium are briefly reviewed. A high resolution hemispherical electron spectrometer with a position sensitive detector, developed for the study of low energy differential scattering from metastable excited states is discussed in detail. This assisted in the measurement of excitation processes characterised by extremely low count rates. The first results of angular distributions for electrons superelastically scattered from the 21,3S states at incident energies of 5, 10 and 30 eV are presented. They have been normalised in magnitude to the convergent close coupling calculation. Cross sections from a 29-state R-matrix calculation and the first order many body theory calculation are also included for comparison. These measurements are in excellent agreement with the convergent close coupling calculations of Bray et al. (1994). 28 refs., 4 figs