Surface-enhanced Raman scattering selection rules

Gao, X., Davies, J.P., and Weaver, M.J. (1990) A test of surface selection rules for surface-enhanced Raman scattering the orientation of adsorbed benzene and monosubstituted benzenes on gold. Journal of Physical Chemistrv, 94, 6858-6864. 

The phenomenon of surface-enhanced infrared absorption (SEIRA) spectroscopy involves the intensity enhancement of vibrational bands of adsorbates that usually bond through contain carboxylic acid or thiol groups onto thin nanoparticulate metallic films that have been deposited on an appropriate substrate. SEIRA spectra obey the surface selection rule in the same way as reflection-absorption spectra of thin films on smooth metal substrates. When the metal nanoparticles become in close contact, i.e., start to exceed the percolation limit, the bands in the adsorbate spectra start to assume a dispersive shape. Unlike surface-enhanced Raman scattering, which is usually only observed with silver, gold and, albeit less frequently, copper, SEIRA is observed with most metals, including platinum and even zinc. The mechanism of SEIRA is still being discussed but the enhancement and shape of the bands is best modeled by the Bruggeman representation of effective medium theory with plasmonic mechanism pla dng a relatively minor role. At the end of this report, three applications of SEIRA, namely spectroelectrochemical measurements, the fabrication of sensors, and biochemical applications, are discussed. 

The revolution in Raman spectroscopy has been slow to come to the college chemistry classroom and laboratory. Standard undergraduate textbooks attempt to cover modern Raman spectroscopy, but achieve mixed results. Textbooks typically devote far less space to Raman scattering than to infrared absorption. The student is often left with the impression that Raman spectroscopy is an esoteric branch of vibrational spectroscopy, useful only for its selection rules or for measurements in aqueous solution. Almost entirely missing is a sense of excitement over such contemporary topics as Raman microscopy and Raman imaging, ultrasensitive surface-enhanced Raman spectroscopy, or industrial process control, and the many other applications enabled by fiber-optic probes. 

Some surface geometries (rough surfaces) concentrate the electric fields of incident light sufficiently to enhance the Raman scattering cross-section so that it is surface sensitive. This gives information on surface vibrational modes, and some information on geometry via selection rules. 

Skoog et al. [2] emphasized the instmmentation advances of the 1980s and early 1990s. Both Fourier-transform Raman (FT-Raman) spectrometers and single-stage spectrographs are discussed. There is some discussion of optical fiber probes, but none of the Raman microprobe. The authors sketch the theory of Raman scattering and present a classical (polarizability derivative) treatment of selection rules and intensity. Resonance enhancement and surface enhancement are treated briefly. In a textbook noted for its emphasis on instrumentation, there is little discussion of current applications. 

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