The simultaneous single-electron capture and production of doubly charged He ions in single collisions between fast protons and He atoms has been studied experimentally as a function of scattering angle (0.3-3.2 mrad) and impact energy (200-500 keV). The angular dependences reveal a critical angle at which the probability for production of ${\mathrm{He}}^{++}$ simultaneously with electron capture peaks sharply. The peak itself as well as the energy dependence of the size and shape of it may be interpreted qualitatively in terms of a capture mechanism proposed already in 1927 by L. H. Thomas on the basis of classical mechanics.

• Received 5 May 1986

DOI:https://doi.org/10.1103/PhysRevLett.57.1414