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Hollow atoms (discovered in 1990 by a French team of researchers around Jean-Pierre Briand) are short-lived multiply excited neutralatoms which carry a large part of their Z electrons (Z ... projectile nuclear charge) in high-n levels while inner shells remain (transiently) empty. The hollow atoms are exotic atomic species whose all, or most, electrons lie in excited states, while the innermost shells are empty.[1] These atomic species were first observed during the interaction of highly charged ions with surfaces.[2]population inversion arises for typically 100femtoseconds during theinteraction of a slowhighly 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 softX-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 potentialsputtering and the formation ofnanostructures on a surface.