An (e,2e) study of double excitation-autoionization mechanisms in helium

Measurements are reported of the triple differential cross sections for autoionization of the (2p 2 ) 1 D and (2s2p) 1 P states of helium, following double excitation by 200 eV electrons and for a 12 degrees scattered electron angle. The direct ionization cross section for 34.7 eV ejected electrons has also been measured. The use of a multiparameter spectrometer has enabled reliable high resolution, 80 meV, data to be obtained. The resonant ionization cross sections of the autoionizing states obtained from the coincidence ejected electron spectra are analysed using the Shore parametrization and compared with other recent experimental data. Possible theoretical models of the double excitation-autoionization process are discussed and the relative importance of electron correlation and double hit mechanisms assessed in view of the available data.