The adsorption of bromide on an Ag(001) electrode has been investigated using in situ x-ray scattering methods. With increasing potential, the bromide undergoes a second order phase transition from a lattice gas to an ordered $c(2\times 2)$ structure. The order parameter is consistent with the 2D Ising model prediction $\beta =1/8\mathrm{}$. A comparison of x-ray and electrochemical measurements indicates significant lateral disorder at low coverages which decreases with increasing coverage.

• Received 25 April 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.1511