State‐selective angular‐differential single‐electron capture in very slow Ar4+‐Ar collisions

State‐resolved angular distributions for single‐electron capture in very‐slow collisions (0.5–5 eV/amu) of Ar4+ and Ar have been measured with a new experimental apparatus. The energy‐gain spectra show that single‐electron capture populates the 4p level, while no capture to 4s is observed in this energy range. The 4p angular distribution has a single maximum close to 0° for energies larger than about 100 eV. At lower energies a second maximum develops and forward scattering is suppressed. The secondary maximum moves to larger scattering angles as the collision energy is lowered and below about 60 eV a third maximum appears. The positions of the primary and the secondary peaks as functions of the collision energy are reproduced by semiclassical calculations of the angular differential cross section.