In a capture reaction the momentum change of the projectile is compensated by the recoiling target ion. The transverse recoil ion momentum is equivalent to the projectile scattering angle the longitudinal recoil momentum reflects the energy loss or gain of the projectile. Extremely high resolution can be reached by COLTRIMS.

Longitudinal recoil ion momentum distribution from the reaction 0.25MeV He²⁺ + He -> He⁺ + He⁺ (for details see mergel94prl)Capture to the n=1 and n=2 level are clearly resolved. Similar experiment reporting on double capture can be found in (doerner98pra). This is the first experiment where the discrete lines have mean seen in a COLTRIMS experiment. Much higher resolution has been achieved in later experiments (see e.g. weber01prl or doernerxxicpeac). A high resolution example for slow collisions is shown here.