X-ray diffraction studies of ordered chloride and bromide monolayers at the Au(111)-solution interface

We present in situ surface x-ray scattering observations of ordered chloride and bromide monolayers on Au(111) electrodes in aqueous solutions as an example of in situ x-ray diffraction from adsorbates of the lighter elements (Z50). At a critical surface density both halides form incommensurate, hexagonal-close-packed monolayers that compress uniformly with increasing potential. Our structural results provide quantitative data on the residual adsorbate charge, the two-dimensional melting, and the two-dimensional compressibilities of halide monolayers.