The crystallography of the GaAs(100)- $c(8\times 2)$-Ga surface, prepared by simultaneous cycles of Ar ion bombardment and annealing at 825 K, has been determined by a low energy electron diffraction $I-V$ analysis. A model consisting of three adjacent dimers and a dimer vacancy yielded the best fit. The main feature of the corresponding structure is the presence of two different types of Ga dimers at the surface. The dimer in the middle is fully dimerized ( $d_{\mathrm{Ga}-\mathrm{Ga}}=2.13\mathrm{}$ Å), while the dimerization for the Ga atoms in both outer dimers is much more subtle ( $d_{\mathrm{Ga}-\mathrm{Ga}}=3.45\mathrm{}$ Å).

• Received 14 February 1995

DOI:https://doi.org/10.1103/PhysRevLett.75.665