Interference between second-harmonic generation from a substrate and from an adsorbate layer

INTRODUCTION In recent years optical second-harmonic generation (SHG) has developed into a useful probe of adsorbate molecules on surfaces.", 2 It has been applied to a wide variety of adsorbate-covered surfaces, ranging from single crystals under ultrahigh vacuum conditions3'4 to liquids and amorphous solids under ambient conditions.5-1" From these experiments it is possible to deduce concentration and orientation of molecular adsorbates 5 as well as to observe desorption, 4 two-dimensional phase transitions,6 and chemical reactions7 of the adsorbed species. Furthermore, SHG from a monolayer of an adsorbate molecular species has been used for measuring second-order nonlinear-optical coefficients of molecules. ' 9 The fact that SHG can be used as a surface probe for such a wide variety of uses stems from the high surface selectivity and specificity inherent in SHG. This is because, as a second-order process, SHG is forbidden under the electricdipole approximation in a medium with central symmetry, but the symmetry is necessarily broken at a surface.'2 Consider, for example, SHG from reflection at an air-solid interface. The effective surface nonlinear polarization responsible for the SHG is given by'