Studies of the threshold electron impact ionization of helium

Studies have been made of the two outgoing electrons in ionization events for which the energy difference E between the total energy and the ionization energy is in the range from 0.2 to 3.0 eV. A coincidence time-of-flight technique has been used to measure their energy distribution and angular correlation functions, and a non-coincidence technique has been used to measure the partial cross section for the production of very slow electrons. The energy distribution function has been found to be uniform in the range of E from 0.2 to 0.8 eV and the width of the angular correlation function has been found to increase with the energy E in a way which is consistent with a E 1/4 dependence. The yield of very low energy electrons is consistent, in the range of E from 0.2 to 1.7 eV, with a total ionization cross section having the energy dependence E n , where n=1.131+or-0.019. These results are all consistent with the Wannier-Peterkop-Rau theory.