SHG enhancement by roughness-induced surface plasmon excitation in alkali-metal overlayers grown on Si(1 1 1)-7 × 7

► K, Rb and Cs adsorbed on Si(1 1 1)-7 × 7 show a large SHG in the coverages above the saturation monolayer. ► SHG spectroscopy shows resonance energies exactly coinciding with the surface plasmon excitation. ► The observed resonance is related to the roughness-coupled excitation of a surface plasmon. ► Custers of identical size observed by STM at RT are discussed to be responsible for the resonance.