Vortices in electron momentum distributions

Recent computations of the electron distributions for proton impact ionization of hydrogen atoms have shown unexpected ``holes,'' which are associated with vortices in the continuum part of time-dependent wavefunctions [1]. We find that the electron distributions calculated in the framework of the Impact-Parameter-Born approximation have the same holes. In contrast, the electron distributions in the Born approximation have no vortices but they appear in the Coulomb Born approximation. \\[4pt] [1] J.H. Macek, J.S. Sternberg, S.Y. Ovchinnikov, T-G. Lee, and D.R. Schultz, submitted for publication.