Near-threshold excitation of metastable helium by electron impact as studied by the time-of-flight method

A time-of-flight method is used to measure the distribution in speed and angle of the metastable atoms produced when ground-state helium atoms are excited by electron impact at near-threshold energies. The results are compared to predictions based on both theoretical and experimental determinations of the differential cross section, and it appears that there are significant discrepancies between the time-of-flight data and the predictions based on available theoretical cross-section calculations.