Second-Harmonic Generation (SHG) from Si(111) and Cu(111) Surfaces

We use the polarization properties of SHG to study the electronic properties of the Si(111)-7x7 surface. By isolating 3 elements of the nonlinear SH susceptibility tensor and noting their change on annealing and oxidizing the surface, we find: 1) SH excited with 1.17 eV radiation is mostly sensitive to an optical transition at 1.7 eV (seen in EELS); 2) electronic wavefunctions linked in the transition lose 3m symmetry in the surface plane when the surface loses the 7x7 structure; 3) it takes 3 X less oxygen exposure to remove the 3m-symmetric part of the surface states than the isotropic part of these states.