COVERAGE-DEPENDENT CHANGES IN THE AZIMUTHAL ANISOTROPY OF 2ND-HARMONIC GENERATED FROM NA/SI(111)7X7 INTERFACES

The azimuthal anisotropy of the second-harmonic generation from a sodium/silicon(111) interface has been probed in an UHV apparatus with 80 femtosecond laser pulses at lambda = 570 nm. As the clean Si(111) 7 x 7 surface (T-s = 300 K) is covered by Na in the range up to one monolayer the initial Si-induced symmetry is observed to change significantly due to an increasing isotropic component in the effective surface nonlinearity. This effect is observed with s-polarized incident and p-polarized nonlinearly reflected radiation and can be reproduced quantitatively assuming a linear increase in the isotropic Na induced component with increasing coverage.