X-ray emission of hollow atoms formed by highly charged argon and xenon ions below a beryllium surface

The X-ray emission induced by highly charged argon and xenon ions impinging on a beryllium surface is investigated. It is found that spectra of the X-ray induced by Ar17,18+ interacting with the surface are very different from those of the X-ray induced by Ar17,18+ interacting with residual gases. The result provides an experimental evidence for the existence of hollow atoms below the surface. Several unexpected X-ray lines are also found in the experiment. Firstly, K X-rays are observed when Ar16+ ions which initially have no K shell holes interact with the surface. Secondly, if there are more than 2 M shell vacancies at the initial time, strong Mαα two-electron-one-photon (TEOP) transitions are found in the collisions of Xe28+,29+,30+ ions with the surface.