We show with both experiment and theory that a Bragg reflection from a crystal surface excited under grazing-incidence conditions is displaced from its reciprocal-lattice point. This new phenomenon allows for the first time a confirmation of the penetration depth of the scattered intensity on the scale of a lattice constant. The experiments presented show the possibility of a controlled depth probing of near-surface atomic correlations.

• Received 13 September 1985

DOI:https://doi.org/10.1103/PhysRevLett.56.1144