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Scanning tunneling microscopy on Ar ion bombarded and annealed aluminum surfaces shows electron interference between the surface and subsurface bubbles of implanted gas. The depth of the bubbles as determined from the energy dependence of the standing waves indicates a minimum around 6--7 layers on Al(111). The appearance and energy dependence of the interference pattern is in good agreement with scattering theory based on free electrons, and indicates the bubbles have a shape given by the Wulff construction.
Schmid et al. (1996) studied this question.