We have measured projectile-energy-loss spectra for 50-, 100-, and 150-keV p+He collisions. From the data we obtained differential double-excitation cross sections as a function of projectile scattering angle. At 150 keV a pronounced peak structure was observed at about 0.7 mrad for double excitation to the (2p2) 1D and (2s2p) 1P states. Our data provide indications for the dominance of a first-order mechanism involving the electron-electron interaction in double excitation for 150 keV at small scattering angles. At lower projectile energies and larger scattering angles a second-order mechanism appears to be of the same order of magnitude as the first-order mechanism. In these regimes, interference effects between the first- and second-order mechanisms could be important.
M. Schulz et al., "Differential Double-Excitation Cross Sections in 50-150-keV Proton-Helium Collisions," Physical Review A, vol. 51, no. 3, pp. 2140-2150, American Physical Society (APS), Mar 1995.
The definitive version is available at http://dx.doi.org/10.1103/PhysRevA.51.2140
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