Surface-enhanced Raman scattering

© 2007 American Institute of Physics, S-0031-9228-0711-020-6 When light interacts with matter, it can scatter inelastically from vibrational quantum states. During that process, photons may lose energy to, or gain it from, vibrational excitations. A change in the photon energy must produce a concomitant shift in the frequency of the scattered light (see box 1). The phenomenon, called the Raman effect, was experimentally discovered in 1928 by C. V. Raman and K. S. Krishnan in India and, independently, by Leonid Mandelstam and Grigory Landsberg in the former Soviet Union.1,2 Raman scattering provides a rich variety of information on the structure and composition of matter, based on its vibrational fingerprints. The vibrational information, which usually occurs at IR frequencies, can be obtained by monitoring the frequency shifts between excitation and scattered light. As a scattering process, however, the Raman effect is exceedingly weak: Typical Raman cross sections per molecule range between 10−30 and 10−25 cm2, with the larger values occurring during resonant Raman conditions, when the frequency of light happens to match an electronic transition in the molecule. By comparison, fluorescence spectroscopy, based on the absorption and emission of light, exploits effective cross sections between 10−17 cm2 and 10−16 cm2. The advent of the laser as an intense and monochromatic source of excitation light was a milestone in the history of Raman spectroscopy and dramatically improved scattering signals. Still, spectroscopists remained hampered by small Surface-enhanced Raman scattering