Double ionization of helium by fast ion impact:: Reexamination of the correlation function

The recently introduced correlation function [M. Schulz et al., Phys. Rev. Lett. 84, 863 (2000)] for the analysis of double ionization processes is reconsidered. We present a model based on the frozen-correlation and continuum distorted-wave with eikonal initial-state approximations, and compare numerical results with experimental data for double ionization in 100 MeV/amu C6+-He and 3.6 MeV/amu Au53+-He collisions. Our calculations confirm earlier conclusions about the primary and secondary roles of final-state and initial-state correlations, respectively. However, we show that the usual definition of the correlation function on the level of cross sections is problematic, since even a fully uncorrelated calculation gives rise to a nontrivial result. The intended limit of a zero-valued function for an independent electron calculation is achieved only if the analysis is performed on the level of impact-parameter dependent transition probabilities. Consequently, the workings of initial- and final-state correlations are reflected unambiguously only on this level.