Electron correlation in ion-atom collisions

We study the role of electron correlation during a high-energy ion-atom collision (dynamic correlation). We show how to define a reference calculation in which correlation is not included during the collision whereas it is included in the initial and final states (frozen-correlation approximation). Correlation may be characterized by a typical time t(corr). The frozen-correlation approximation corresponds to the form of the transition amplitude when assuming that t(corr) is much larger than the collision time. We apply our theory to double excitation and single ionization of helium and show that dynamic correlation may play a more important role in ionization than in double excitation.