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Summer 2000 ISSN 1044 5536 Robert C Johnson and Joseph T Hupp Department of Chemistry and Center for Nanofabrication and Molecular Self Assembly Northwestern University Center for Photochemical Sciences Bowling Green State University Bowling Green Ohio 43403 Introduction Second harmonic generation from isotropic solution was first observed over three decades ago 1 While coherent second harmonic generation SHG is cancelled in isotropic media due to symmetry 2 incoherent second harmonic scattering known as hyper Rayleigh scattering HRS is weakly allowed Though HRS lay relatively dormant for the quarter century following its discovery its popularity was revived when it was appreciated that HRS reports directly on the magnitude of the first hyperpolarizability tensor of molecules in solution 3 describes second order nonlinear optical NLO activity an important characteristic of molecules designed for use in emerging technological applications such as photonics the photon based analog of electronics 4 Over the last ten years HRS has emerged as the method of choice for solution phase measurement of HRS circumvents the need to pole the sample electrically a sometimes severe limitation of the more common technique for determining in solution electric field induced second harmonic generation EFISHG This versatility permits investigation of both ionic and non dipolar molecules neither of which are generally accessible to EFISHG experiments Furthermore no other data are needed in order to extract from an HRS experiment in contrast in an EFISHG experiment the analyte s ground state dipole moment and second order hyperpolarizability must be determined or estimated independently in order to determine The information provided by HRS experiments however can be used for more than simply predicting intrinsic frequency doubling efficiencies For example symmetry information regarding optical electron transfer can be deduced by observing the polarization of the HRS signal from



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