Near threshold electron impact excitation functions for the n=2 states of helium by the trapped-electron method

The threshold excitation spectrum of helium is presented and discussed. From a series of trapped electron spectra for increasing well depths, the excitation functions of the four n=2 states of helium are obtained from threshold to above 22eV. The inelastic cross section to 23.5eV is observed and compared with total metastable production measurements.