SIGNAL-TO-NOISE IN RAMAN SPECTROSCOPY

Figure Bl.22.6. Raman spectra in the C-H stretching region from 2-butanol (left frame) and 2-butanethiol (right), each either as bulk liquid (top traces) or adsorbed on a rough silver electrode surface (bottom). An analysis of the relative intensities of the different vibrational modes led to tire proposed adsorption structures depicted in the corresponding panels [53], This example illustrates the usefiilness of Raman spectroscopy for the detennination of adsorption geometries, but also points to its main limitation, namely the need to use rough silver surfaces to achieve adequate signal-to-noise levels.

Nitrophenyl groups covalently bonded to classy carbon and graphite surfaces have been detected and characterized by unenhanced Raman spectroscopy in combination with voltammetry and XPS [4.292]. Difference spectra from glassy carbon with and without nitrophenyl modification contained several Raman bands from the nitrophenyl group with a comparatively large signal-to-noise ratio (Fig. 4.58). Electrochemical modification of the adsorbed monolayer was observed spectrally, because this led to clear changes in the Raman spectrum. 

The use of Raman spectroscopy in the lumber/paper industry has been found to be feasible using the FT-Raman technique. Earlier results using a visible laser were limited due to the laser-induced fluorescence created with most wood samples. Measures to circumvent fluorescence were time-consuming, and the signal-to-noise (S/N) ratio was poor. With most wood samples, using a near-IR laser excitation, fluorescence essentially was eliminated. 

Infrared spectroscopy often achieves high signal-to-noise ratios, but the intense bands of typical oxide supports interfere with much of the interesting spectral region. Raman spectroscopy is an excellent alternative technique in such cases, because many supports exhibit only weak... 

UV-vis spectroscopy of this molybdenum oxide demonstrated an absorption extending from below 580 to 1000 nm, which was attributed to an IVCT transition between fivefold coordinated Mo5+ centers and a neighboring sixfold coordinated Mo6+ center (Dieterle, 2001 Dieterle and Mestl, 2002 Payen et al., 1986). Hence, the variations in the Raman spectra were attributed to oxidation in the laser spot (to explain the new bands obtained during excitation at 244 nm), and also to resonance effects occurring in reduced molybdenum oxides (to explain the high signal-to-noise ratio obtained when the sample was excited at 632 nm). 

Near-infrared excited FT-Raman spectroscopy has recently begun to show promise (Schrader, 1990), because the fluorescence is drastically reduced. It has the Jaquinot advantage over classical Raman spectroscopy, which affords a better signal-to-noise ratio. FT-Raman is an excellent technique to supplement FTIR difference spectroscopy in investigations of intramolecular protein reactions because Raman spectra have the... 

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