Treatment of Linear and Nonlinear Dielectric Property of Molecular Monolayer and Submonolayer with Microscopic Dipole Lattice Model: I. Second Harmonic Generation and Sum-Frequency Generation

摘要

In the currently accepted models of the nonlinear optics, the nonlinearradiation was treated as the result of an infinitesimally thin polarizationsheet layer, and a three layer model was generally employed. The directconsequence of this approach is that an apriori dielectric constant, whichstill does not have a clear definition, has to be assigned to this polarizationlayer. Because the Second Harmonic Generation (SHG) and the Sum-FrequencyGeneration vibrational Spectroscopy (SFG-VS) have been proven as the sensitiveprobes for interfaces with the submonolayer coverage, the treatment based onthe more realistic discrete induced dipole model needs to be developed. Here weshow that following the molecular optics theory approach the SHG, as well asthe SFG-VS, radiation from the monolayer or submonolayer at an interface can berigorously treated as the radiation from an induced dipole lattice at theinterface. In this approach, the introduction of the polarization sheet is nolonger necessary. Therefore, the ambiguity of the unaccounted dielectricconstant of the polarization layer is no longer an issue. Moreover, theanisotropic two dimensional microscopic local field factors can be explicitlyexpressed with the linear polarizability tensors of the interfacial molecules.Based on the planewise dipole sum rule in the molecular monolayer, crucialexperimental tests of this microscopic treatment with SHG and SFG-VS arediscussed. Many puzzles in the literature of surface SHG and SFG spectroscopystudies can also be understood or resolved in this framework. This newtreatment may provide a solid basis for the quantitative analysis in thesurface SHG and SFG studies.