Excitation of the n=2 states of helium by positron impact

A high-resolution (ΔE∼55 □ meV) trap-based positron beam has been used to measure absolute scattering cross sections for the excitation of the resolved 2 S1, P states of helium at energies between threshold and 38 eV. The experimental integral cross sections, which have typical uncertainties of 10% or less, are compared with several theoretical calculations, and the agreement is generally very favorable. In particular, a new convergent close-coupling approach shows excellent agreement with the experimental data.