Cross sections for single electron emission have been measured in collisions of 95-MeV/u ${\mathrm{Ar}}^{18+}$ projectiles with atomic Li for electron energies ranging from 3 to 1000 eV and angles ranging from $25°$ to $155°.$ Models based on the Born approximation are introduced to separate two- and three-body effects in the angular distributions of the ejected electrons. Both experiment and theory provide information about the separability of the two- and three-body effects. The high projectile velocity and the use of the Li target are shown to be essential for the present analysis. The emission of the $1s$ electron is attributed mainly to three-body effects. The cross section for three-body collisions rapidly decreases with the electronic energy transfer involving a power law with an exponent of $-\mathrm{3.5.}$ Consequently, two-body effects dominate at high electron emission energies. Remarkably large contributions from two-body collisions were also observed for the low-energy emission of the $2s$ electrons. Demonstrating the analogy in ionization by photons and ions, the two- and three-body processes are associated with Compton scattering and photoabsorbtion, respectively.

• Received 19 August 1998

DOI:https://doi.org/10.1103/PhysRevA.59.1262