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

In the currently accepted models of the nonlinear optics, the nonlinear radiation was treated as the result of an infinitesimally thin polarization sheet layer, and a three layer model was generally employed. The direct consequence of this approach is that an apriori dielectric constant, which still does not have a clear definition, has to be assigned to this polarization layer. Because the Second Harmonic Generation (SHG) and the Sum-Frequency Generation vibrational Spectroscopy (SFG-VS) have been proven as the sensitive probes for interfaces with the submonolayer coverage, the treatment based on the more realistic discrete induced dipole model needs to be developed. Here we show that following the molecular optics theory approach the SHG, as well as the SFG-VS, radiation from the monolayer or submonolayer at an interface can be rigorously treated as the radiation from an induced dipole lattice at the interface. In this approach, the introduction of the polarization sheet is no longer necessary. Therefore, the ambiguity of the unaccounted dielectric constant of the polarization layer is no longer an issue. Moreover, the anisotropic two dimensional microscopic local field factors can be explicitly expressed with the linear polarizability tensors of the interfacial molecules. Based on the planewise dipole sum rule in the molecular monolayer, crucial experimental tests of this microscopic treatment with SHG and SFG-VS are discussed. Many puzzles in the literature of surface SHG and SFG spectroscopy studies can also be understood or resolved in this framework. This new treatment may provide a solid basis for the quantitative analysis in the surface SHG and SFG studies.Comment: 23 pages, 3 figure

Optical characterisation of anodic sulphide films on Hg1-xCdxTe (MCT) grown by the potential step method

The growth of native sulphide films on Hg1-xCdxTe (MCT) by potential steps to two different electrochemical growth regions has been studied by in-situ ellipsometry, photocurrent spectroscopy and second harmonic generation (SHG) rotational anisotropy. Films grown at -0.4 V versus SCE were porous and consisted mainly of cubic close packed (ccp) Us. The photocurrent spectrum of the film exhibited a cut-off which closely conformed to the expected bandgap of CdS. The gradual increase in SH intensity as the sulphide film thickened was due to a contribution to the overall SH signal from the CdS film itself. No evidence of any hcp species was apparent in the rotational anisotropy patterns, with the four-fold pattern expected for the vicinal MCT surface being maintained even after film growth. When the potential was stepped to -0.3 V. the film grown absorbed the ellipsometer radiation once a thickness of 86 nm was reached. Photocurrent measurements showed a tail extending into the red region of the spectrum. These phenomena have been attributed to the incorporation of ccp HgS into the sulphide film as at this potential, the electrochemical reaction of the HgTe component of MCT can occur. SHG rotational anisotropy confirmed the four-fold symmetry of the ccp surface film. (C) 2003 Elsevier Ltd. All rights reserved.</p