Intermediate-state dynamic correlation in the ionization of helium

Forced impulse method (FIM) calculations of cross sections for the ionization of helium by energetic ions of various charge are analysed by studying intermediate-state dynamic correlation as it reveals the role of the interelectron interaction, V(ee) during the collision. In the FIM, the collision time is cut into sub-intervals, or segments. If these are short enough, it is supposed that the time development of the system for the segment can be described in terms of an independent particle model U-matrix in which V(ee) is turned off. However, knowledge of fully correlated eigenstates at the beginning and end of each segment is required to implement the method. If the collision time is sufficiently short, dynamic correlation in just the initial and final states is all that is needed. For slower collisions multiple cuts may be required; this introduces intermediate-state dynamic correlation. In this paper we discuss the placements of the cuts in relation to a correlation length. We introduce a useful new concept, pliability, an action that allows the prediction of the cut placements needed and characterizes the collision. We present results with the sign of V(ee) reversed to investigate the role of the mean field.