Abstract
We have measured projectile-energy-loss spectra for 50-, 100-, and 150-keV [ital 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 (2[ital p][sup 2]) [sup 1][ital D] and (2[ital s]2[ital p]) [sup 1][ital P] 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.
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