Surface Raman Spectroscopy without Field Enhancement

SURFACE RAMAN SPECTROSCOPY 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. 

The molecular orbitals of the adsorbate and the electronic band structure of the substrate may be complex and often poorly understood. So predicting interactions between the two is nontrivial and inexact. Stated generally, the adsorbate-substrate complex has a different electron distribution from the isolated components, resulting in a different cross section. While the details are usually complex and often undefined, at least one theory of the effects of adsorption on Raman cross section has yielded useful explanations and predictions. The theory attributes chemical enhancement of cross sections to charge transfer between adsorbate and substrate orbitals (or vice versa) and is generally known as charge-transfer theory (1,15,16). 

Upon adsorption, the orbitals of the metal and adsorbate interact to some degree, as required to form a metal-adsorbate bond. Overlap of metal and adsorbate orbitals permits electrons to be transferred from the molecule to the 

The application of confocal Raman microscopy to surface Raman has led to some promising results for transition-metal surfaces that yield weak or negligible field enhancement. High-performance CCD detection provided high SNR, and the confocal sampling reduced the background from the solution or gas over the surface (22, 23). Metals such as platinum, rhodium, and ruthenium 

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SURFACE RAMAN SPECTROSCOPY WITHOUT FIELD ENHANCEMENT... 

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). 

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. 

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