Differential Double-Excitation Cross Sections in 50-150-keV Proton-Helium Collisions

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 (2p ² ) ¹ D and (2s2p) ¹ 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.