EXPERIMENTAL-EVIDENCE OF THE ORIGIN OF ROTATIONAL ANISOTROPY IN 2ND-HARMONIC GENERATION FROM VICINAL AL SURFACES

Three possible mechanisms of the rotational anisotropy of second harmo nic generation at metal surfaces are discussed, namely, (i) the anisot ropy of the bulk (quadrupole-allowed) nonlinear currents, (ii) the eff ect of the outer surface contour, and (iii) the surface-induced anisot ropic response from non-symmetrically stacked lattice planes underneat h the geometrical surface. Unambiguous experimental verification of th e three mechanisms on oxide-covered vicinal Al(001) and vicinal Al(111 ) surfaces is obtained and provide clear evidence that the latter effe ct plays the major role. The possibility of examining the origin of th e second harmonic generation anisotropy on silver surfaces is discusse d.