The structure of a type of surface magic cluster is determined by a combination of scanning tunneling microscopy, density-functional calculations, and dynamical low energy electron diffraction. The diffraction method is applicable because these clusters created through hierarchical self-organization of Ga deposited onto a $\mathrm{S}\mathrm{i}(111)\mathrm{\text{−}}7\times 7$ surface have identical size and structure and form an ordered array with exact translational symmetry. The unprecedented detailed structure information provided by the diffraction measurement is consistent with direct microscopic imaging and theoretical calculations.

• Received 24 July 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.066103