Using a continuous-wave technique we have observed resonant optical second-harmonic generation from an electrodeposited Cd overlayer on Ag(111) at coverages near one monolayer while the sample was under electrochemical control. Optical polarization and electrochemical evidence suggest that the overlayer forms in three steps, each associated with one-third of a monolayer and having $(\sqrt{3}\times \sqrt{3}){R30\mathrm{}}^{\circ }$ symmetry. The overlayer is epitaxial and buckled below two monolayers. However, above two monolayers the entire overlayer becomes incommensurate and close packed.

• Received 8 June 1987

DOI:https://doi.org/10.1103/PhysRevB.37.1028