Double-electron-capture cross sections for Li{sup 3+} collisions with He at intermediate-to-high velocities

We have measured double-electron capture cross sections for Li{sup 3+} projectiles and He targets in the 2.0-5.5 MeV energy range (3.4-5.6 a.u. velocity range). Double-electron-capture cross sections for this collision system were measured before, but only at relatively lower energies. There is a good agreement between the three available sets of experimental data for double capture at the lower end of the present data energy interval, i.e., where the ranges from different measurements overlap. The present measurements for other collision channels that include single and double ionization, single capture, and transfer ionization corroborate the reliability of the data normalization adopted in our work. Our experimental data for double capture are compared with the corresponding theories in order to assess the relative performance of the main two strategies, the semiclassical independent particle model, and the full quantum-mechanical four-body theories. We thoroughly discuss possible reasons for the discrepancies among various theoretical predictions from these two frameworks and point out several potential paths for their improvement.