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Surface states as probes of buried impurities

Shockley surface states at metal surfaces decay exponentially into the crystal, typically penetrating several interplanar spacings. Within this distance defects such as impurity atoms will scatter the surface state electrons and give rise to characteristic standing wave patterns in the local density of states outside the surface, similar to those recently detected with the STM due to scattering by adatoms. A multiple-scattering theory is developed to model this situation and applied to impurities near the Cu(111) surface. The results are discussed within a two-dimensional scattering model and the prospects for observation with the STM considered.