Benchmark Experiment and Theory for Near-Threshold Excitation of Helium by Electron Impact.*

Experimental measurements of absolute angle-differential scattering cross sections for electron-impact excitation of the n=2,3 states of helium, obtained with a new time-of-flight technique for incident energies below 24 eV, are presented. The results are compared with state-of-the-art calculations employing the R-matrix and convergent close-coupling approaches. The agreement between experiment and theory is generally very good. A detailed analysis of the theoretical predictions highlights the need for multi-configuration expansions of the target states, as well as coupling to the ionization continuum, in order to achieve a highly accurate theoretical description of the problem.