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Surface-enhanced hyper-Raman and resonance Raman scattering Raman spectroscopy as an analytical technique has always suffered from the disadvantage of relatively small signals. Even under conditions of electronic resonance, Raman scattering is normally much weaker than fluorescence and has not been able to compete with fluorescence-based techniques when high sensitivity is required, despite the much greater information content of Raman spectra. Therefore, great excitement accompanied the discovery in the 1970’s that Raman scattering from molecules adsorbed to roughened surfaces of coinage metals can be enhanced by many orders of magnitude. The phenomenon of surface enhanced Raman scattering (SERS) is believed to arise largely from enhancement of the electromagnetic field near the metal surface by excitation of localized surface plasmons. The magnitude of the effect depends on the length scale of the surface roughness relative to the wavelength of the light as well as the wavelength-dependent complex dielectric function of the metal. We have focused particularly on using surface plasmon resonance to enhance signal strengths in hyper-Raman scattering, the two-photon analog of ordinary Raman. Hyper-Raman offers potential advantages over linear Raman as an analytical technique including greater selectivity, less interference from the laser line, ease of observing low-frequency vibrations, less photochemical damage with the long-wavelength excitation laser, and better resolution with lower backgrounds in imaging applications. The main problem with hyper-Raman is its extreme weakness, but it can be greatly enhanced by coupling to surface plasmons. We have obtained hyper-Raman spectra of several molecules on silver nanoparticle films using laser powers below 100 μW, about 3 orders of magnitude lower than used in previous hyper-Raman studies.
We are also initiating studies directed toward better understanding the fundamental physics of the surface-enhanced Raman phenomenon, particularly with electronically resonant chromophores. While the relationship between ordinary linear absorption spectra and resonance Raman excitation profiles is well understood for free molecules in solution, the situation is not so simple under conditions that produce surface enhanced resonance Raman spectra (SERRS). We have developed a theoretical approach to optical absorption and SERRS that treats the surface enhancement as arising from transition-dipole coupling between the near-degenerate electronic transitions on the nanoparticle and on the molecule, and are initiating experiments to test these predictions. The results of these studies should be relevant not only to SERS and SERRS but also to other plasmon-enhanced spectroscopies such as surface enhanced fluorescence and surface enhanced fluorescence resonance energy transfer (FRET). Recent publications on this topic Weinan Leng, Han Young Woo, Doojin Vak, Guillermo C. Bazan, and Anne Myers Kelley. Surface-enhanced resonance Raman and hyper-Raman spectroscopy of water-soluble substituted stilbene and distyrylbenzene chromophores. J. Raman Spectrosc. 37, 132-141 (2006). Anne Myers Kelley. A molecular spectroscopic view of surface plasmon enhanced resonance Raman scattering. J. Chem. Phys. 128, 224702 (2008). |
