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Hollow Atoms (discovered in 1990 by a French team of researchers around Jean-Pierre Briand) are short-lived multiply excited neutral atoms which carry a large part of their Z electrons (Z ... projectile nuclear charge) in high-n levels while inner shells remain (transiently) empty. This population inversion arises for typically 100 femtoseconds during the interaction of a slow highly charged ion (HCI) with a solid surface.
Despite this limited lifetime, the formation and decay of a hollow atom 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 hollow atom may also cause the release of target atoms and -ions via potential sputtering and the formation of nanostructures on a surface.