Volume 13 . Issue 2 Summer 2000 . ISSN 1044-5536IntroductionSecond harmonic generation from isotropic solution was first observed over three decades ago.1While coherent second harmonic generation (SHG) is cancelled in isotropic media due tosymmetry,2 incoherent second harmonic scattering, known as hyper-Rayleigh scattering (HRS), isweakly allowed. Though HRS lay relatively dormant for the quarter-century following its discov-ery, its popularity was revived when it was appreciated that HRS reports directly on the magnitudeof the first hyperpolarizability tensor, β, of molecules in solution.3 β describes second-order nonlin-ear optical (NLO) activity, an important characteristic of molecules designed for use in emergingtechnological applications such as photonics, the photon-based analog of electronics.4 Over the lastten years, HRS has emerged as the method of choice for solution phase measurement of β. HRScircumvents the need to pole the sample electrically, a sometimes severe limitation of the morecommon technique for determining β in solution, electric-field-induced second harmonic genera-tion (EFISHG). This versatility permits investigation of both ionic and non-dipolar molecules, nei-ther of which are generally accessible to EFISHG experiments. Furthermore, no other data are neededin order to extract β from an HRS experiment; in contrast, in an EFISHG experiment the analyte’sground state dipole moment, µ, and second order hyperpolarizability, γ, must be determined (orestimated) independently in order to determine β.The information provided by HRS experiments, however, can be used for more than simply pre-dicting intrinsic frequency doubling efficiencies. For example, symmetry information regarding op-tical electron transfer can be deduced by observing the polarization of the HRS signal from charge-transfer chromophores under resonance or near-resonance conditions. HRS has also been applied tocolloidal solutions of nanoparticles, an increasingly popular class of materials with interesting opti-cal properties. The experiment’s unique characteristics allow interrogation of particle/solution in-terfaces that are invisible to conventional SHG measurements. HRS studies of colloidal metalnanoparticles have revealed enormous hyperpolarizabilities resulting from partial resonance withthe colloid’s intense surface plasmon absorption bands. Further experiments with metal particleshave yielded symmetry information about particle clusters and aggregates.Experimental ImplementationIn an HRS experiment, laser light of frequency ω is directed into an isotropic sample and theincoherently scattered second harmonic (i.e. frequency-doubled) light of frequency 2ω is collected(Scheme 1). This scattered light is HRS. The microscopic origin of the scattering is in the firsthyperpolarizability tensor, βijk, which is the coefficient of the second-order term in the expansionof the field-induced dipole moment (eq 1). In this expression, α and γ represent the polarizabilityand second hyperpolarizability tensors, respectively; the indices i, j, k, l, etc. represent molecularcoordinates; and E represents the incident field strength. Repeated indices imply summation overthose indices.µind = αijEj + βijkEjEk + γijklEjEkEl + ...Continued on page 3Hyper-Rayleigh Scattering: A Spectroscopic Tool for NonlinearOptical Property Characterization, Charge Transfer SymmetryInvestigation, and Nanoscale Interface InterrogationRobert C. Johnson and Joseph T. HuppDepartment of Chemistry and Center for Nanofabrication and Molecular Self-AssemblyNorthwestern University(1)The Spectrum©Center for Photochemical Sciences . Bowling Green State University . Bowling Green . Ohio . 43403From the Executive DirectorD. C. Neckers, Executive Director, Center for Photochemical Sciences, Bowling Green State UniversityThe Spectrum Page 2In This IssueHyper-Rayleigh Scattering: A Spectroscopic Tool for Nonlinear Optical Property Characterization,Charge Transfer Symmetry Investigation, and Nanoscale Interface Interrogation .......................................................1From the Executive Director.................................................................................................................................................2Photosensitized Oxygenation of Small Ring Olefins ........................................................................................................9Something New in Transition Metal Complex Sensitizers: Bringing Metal Diimine Complexes andAromatic Hydrocarbons Together .....................................................................................................................................17Center for Photochemical Sciences Publications ...........................................................................................................22As editor of the Journal of the American Chemical Society, Cheves Walling had more than the usual cadre of enemies.For one thing, he wanted to eliminate the “Communications to the Editor”. “Publish the experimental details,” heargued, “or don’t publish the work at all.” In addition, he said, facetiously but semiseriously, that the experimentalsections of journals should be published in archival form, and the discussion sections published on paper that dis-solved after five or ten years. Melvin S. Newman, another organic chemist of the same era, said it in another way, “Ifyou make a mistake, make it in the discussion section; never report an incorrect experimental result.”On a recent visit to Kodak, Samir Farid and I were talking about how poly(vinyl cinnamates) became the firstcommercial photoresists. Louis Minsk discovered that these soluble polymers were converted to insoluble solids withultraviolet light. The first patents appeared in the early 1950s. Poly(vinyl cinnamates) became the first photochemicalroute to an imaged pattern that later became a semiconductor surface. Minsk, himself, believed that the solidificationcame about because he was initiating a chain polymerization in the cinnamate that led to crosslinking. Arnost Reiser,who has written on more than one occasion for The Spectrum, was a staff scientist at Kodak/Harrow (England) at thetime, and ferreted out all of the products in the Minsk poly(vinyl cinnamate) system. He showed, using careful stud-ies of small compounds, that the likely reason for solidifying poly(vinyl cinnamates) into an insoluble solid wasbecause