Ejected electron spectroscopy of autoionizing states excited by low energy electron impact

The autoionizing states of helium that exist beneath the n=2 threshold of He + have been extensively investigated using a low energy (10-200 eV) electron impact spectrometer. The instrument was purpose-built for studying the electron ejection spectra of autoionizing states and features a high intensity ( approximately 1 mu A) incident beam and a high resolution ( approximately 40 meV) electron energy analyser. The energies of a number of the autoionizing states of helium have been measured, and our values are compared with previous results of theory and experiment. In a few cases the resolution of the instrument has allowed us to estimate the widths of the very short-lived states. An outstanding feature of the spectra obtained at energies just above the thresholds for excitation of the autoionizing states is the phenomenon of displaced thresholds.