Surface Raman without field enhancement

Raman spectra of monolayers were first obtained on roughened silver surfaces that exhibit strong field enhancement, since the scanning/photomultiplier tube spectrometers of the time needed the gain in sensitivity of a factor of to provide useful spectra (3,4). Multichannel spectrometers permitted spectrum acquisition without field enhancement about 8 years later (12), for the reasons discussed in Section 13.2. Surface Raman without field enhancement is conceptually simpler, so it will be discussed first. Section 13.5 describes the requirements and additional benefits when field enhancement occurs. 

An early example of surface Raman without field enhancement is shown in Figure 13.7. This measurement predated the development of CCD detectors and represents a major achievement in terms of sensitivity (12). The adsorption of nitrobenzene on a well-defined nickel surface was carried out in UHV then Raman scattering was observed with a single spectrograph and an intensified Vidicon detector. Unenhanced spectra were obtained from 7.5 x lO molecules cm of nitrobenzene (1.1 x 10 " mol cm ), corresponding to a submonolayer. Examples of surface Raman spectroscopy with resonance enhancement include monolayers of phthalocyanines on gold (17) and ordered graphite (18), and multilayers of metalloporphyrins (19). 

SURFACE RAMAN SPECTROSCOPY WITHOUT FIELD ENHANCEMENT... 

In this paper, we report on simultaneous consideration of incident field enhancement and local density of photon states enhancement near a metal particle with spherical shape as a reasonable primary model for single molecule Raman spectroscopy. Joint action of these two factors at the same point of space is found to offer up to lO -fold enhancement of Raman scattering rate. To the best of our knowledge this is the first evidence that consistent theory of single molecule Raman spectroscopy and comprehensive description of so-called hot points in surface enhanced spectroscopies can be constructed without necessarily involvement of chemical mechanisms. 

From the perspective of chemical analysis, the ability to observe spectra from surface monolayers without requiring field enhancement demonstrates the high sensitivity of modem Raman instramentation. In addition, it indicates that multilayer films such as polymer coatings, lubricants, or metal oxides are readily observable on a variety of substrates, provided the multilayer is reasonably transparent. However, the sensitivity constraints in the absence of field enhancement are generally severe when dealing with monolayers,... 

Raman enhancement by SERS is mainly attributed to an electromagnetic (EM) field enhancement via localized optical fields of the metallic nanostmctures that are related to plasmon resonance excitation. The increase of the cross-section with contact between the metal nanostructure and a molecule induces an additional enhancement. Without enhancement by the electrical resonance between incident light and molecules, the total Stokes-Raman signal P (vs) is proportional to the number of molecules in the scattering volume N, the Raman cross-section without surface enhancement and the excitation laser intensity... 

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