Two-centre effects in ejected-electron spectra from ionization of helium atoms in collisions with fast, fully stripped ions

A joint experimental and theoretical study of the double-differential cross sections (DDCS) for ejection of electrons from helium by 1.00 and 1.84 MeV amu-1 p+, He2+, C6+ and O8+ is presented. Electron-emission angles from 20' to 160' are surveyed, and the electron energies range from 4 to 2500 eV. Single-differential and total cross sections, obtained by integrating the DDCS over angle and electron energy, are also presented. Large departures from the q2 dependence (where q is the projectile charge) predicted by the first Born approximation are observed in the cross sections. The deviations, which are seen to increase with projectile charge are discussed and compared with calculations, using the first Born approximation and a continuum-distorted-wave eikonal-initial-state model.