AN (E, 2E) STUDY OF DOUBLE-EXCITATION AUTOIONIZATION MECHANISMS IN HELIUM

Measurements are reported of the triple differential cross sections fo r autoionization of the (2p2) 1D and (2s2p) 1P states of helium, follo wing double excitation by 200 eV electrons and for a 12-degrees scatte red electron angle. The direct ionization cross section for 34.7 eV ej ected electrons has also been measured. The use of a multiparameter sp ectrometer has enabled reliable high resolution, 80 meV, data to be ob tained. The resonant ionization cross sections of the autoionizing sta tes obtained from the coincidence ejected electron spectra are analyse d using the Shore parametrization and compared with other recent exper imental data. Possible theoretical models of the double excitation-aut oionization process are discussed and the relative importance of elect ron correlation and double hit mechanisms assessed in view of the avai lable data.