Double excitation of helium by 3 MeV proton impact: experiment and theory

Doubly differential cross sections for the double excitation of helium by high velocity protons are measured and compared with the predictions of the first-order Born approximation. Preliminary experimental data for the shapes and intensities of the resonances 2s2 1S, 2p2 1D and 2s2p 1P have been obtained from high resolution electron spectra at 20', 90' and 150' for a proton energy of 3 MeV. Both experiment and theory show that the excitation of the 1P resonance largely exceeds the 1D and 1S ones at this collision velocity. The shape and emission yield of the 1P line is well described by theory over the whole angular range. The agreement for the other two resonances is not always satisfactory.