STRUCTURE AND STABILITY OF UNDERPOTENTIALLY DEPOSITED LAYERS ON AU(111) STUDIED BY OPTICAL 2ND HARMONIC-GENERATION

This report details changes in the second harmonic optical response from Au(111) during the underpotential deposition of Cu, Ag, Pb, and Tl. The signal from Au(111) exhibits a strong anisotropy that is assigned to a coupling of the SH photons with interband transitions. Underpotential deposition is used to form overlayers of foreign metals with precise control of surface adatom concentration. The participation of the d-like electronic states in binding of metal adsorbates is evidenced in the optical response. We compare the results from the Au(111)/Cu and the Au(111)/Ag interface with those from the (111) surface of the single-crystal adsorbates and find that the results are consistent with the known electronic structure of the metals and the geometric structures of the overlayers. In the case of Pb and Tl deposition, a strong perturbation of anisotropic second harmonic signal is observed and attributed to the inability of the large adatoms to lattice match with the substrate. The consideration of domain effects is important for the optical results to be consistent with recent structural determinations. The results are compared with related studies of Pb deposition on Cu(111). The use of second harmonic generation to monitor the stability of the overlayer in the absence of an applied field is also demonstrated.