Hollow atoms

The `hollow atom' (HA) is the latest and probably most exotic creation of atomic collision physics. HA are short-lived multiply-excited neutral atoms which carry a large part of their Z electrons (Z, the projectile nuclear charge) in high-n levels while inner shells remain transiently empty. This population inversion arises for typically 100 fs during the interaction of a slow highly charged ion with a solid surface. Despite this limited lifetime, the formation and decay of a HA can be conveniently studied from ejected electrons and soft x-rays, and the trajectories, energy loss and final charge state distribution of surface-scattered projectiles. For impact on insulator surfaces the potential energy contained by HA may also cause the release of target atoms and ions. This topical review gives a short historical account of relevant experimental methods and studies in this field, presents a now widely accepted scenario for HA formation and decay, discusses some results from recent studies of the authors and concludes with an outlook on open questions and further promising aspects in this new field of atomic collisions.