Excitation of Metastable Helium Atoms by Positron Impact

We present measurements of the excitation of the 2¹S metastable state of helium by positron impact at energies between the threshold for excitation (20.6 eV) and 40 eV. The present results are obtained by directly measuring the yield of excited atoms that result from crossing a low-energy, high-resolution, magnetically-confined and pulsed positron beam from a buffer gas trap with an effusive beam of He atoms. The measured excitation function is compared with previous measurements which used a scattering cell, and with a recent state-of-the-art theoretical calculation.