A measurement of the electron impact ionization cross section of helium atoms in metastable states

Cross sections for the ionization of metastable helium atoms, predominantly in the 23S state, at electron energies from the threshold to 1000 eV have been measured using the crossed electron and fast atom beam technique. Corrections are made for the presence of ground-state atoms and a very small concentration of highly excited atoms in the beam. Good agreement is observed with earlier measurements which were restricted to energies below the ionization threshold of the ground state. The measured cross sections are compared with various theoretical data. (author)