DIFFERENTIAL DOUBLE-EXCITATION CROSS-SECTIONS IN PROTON-HELIUM COLLISIONS STUDIED BY ENERGY-LOSS SPECTROSCOPY

We have measured ion energy-loss spectra for 150 keV proton-helium col lisions as a function of the projectile scattering angle. From the dat a we obtained double excitation cross sections differential in the pro ton scattering angle as well as the ratios of both double excitation a nd single ionization to single excitation. In these ratios pronounced peak structures are observed at about 0.7 mrad. Two alternative interp retations of these peak structures are offered: They may be due to bin ary collisions between the projectile and the target electrons, or the y could be a manifestation of an interference between different transi tion amplitudes leading to the same final state of the collision.