Selection Rules for SERS

According to the classical model of the Raman process, the electromagnetic field of the exciting light induces a dipole moment, jjl, in the molecule which is proportional to the electric field E. The proportionality factor, a, which is called the electric polarizability is 

Raman vibrational selection rules are related to the symmetry of the polarizability components. Considering only the fundamental vibrations 

In this expression, cr, p = x, y, or z, where a p is one of the polarizability components and (i and f) are the initial and final vibrational wave functions. Each of the factors in Eq. (80) belongs to an irreducible representation T, and according to group theory, in order for the integral in Eq. (80) to be nonvanishing, the direct product TiXT xTf must contain the totally symmetric representation. Thus, for a fundamental vibration to be Raman active, YfYf must transform under symmetry operations in the same manner as one of the a p components. This is the basis of the normal Raman vibrational selection rule. 

This lowering of symmetry for SERS appears to be the case for the pyrazine system, which, because it has two nitrogens in the 1- and 4-positions of the 

If the second term in this equation can become comparable to the first term 

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By the two strong-enhancement mechanisms as discussed above a number of features inherent in SERS can now be understood. They have been summarized by Creighton (1990). Finally, we notice that the selection rules for SERS are fundamentally the same... 

Surface selection rules for SERS are generally different from that for NRS, as the symmetry of the molecular nuclear and/or electron arrangement may be disturbed or even broken at the surface. Thus, the orientation of the molecule relative to the surface normal is important and can be detected flat versus tilted versus perpendicular orientations give rise to distinct SERS signatures since the various vibrational bands of the adsorbed molecule, e.g., bands due to in plane and out of plane modes of an aromatic compound are differently enhanced according to the corresponding components of the tensor ttmoiecuie-... 

Numerous SERS studies of adsorbed molecules have appeared in the literature. Obviously, it is a useful method for the identification of species at the interface, and its inherent surface sensitivity is an attractive feature. In this context it should be noted that the adsorption of a molecule can change the selection rules for Raman scattering, and modes that are Raman inactive in the isolated molecule may show up in SERS. 

Some characteristics of, and comparisons between, surface-enhanced Raman spectroscopy (SERS) and infrared reflection-absorption spectroscopy (IRRAS) for examining reactive as well as stable electrochemical adsorbates are illustrated by means of selected recent results from our laboratory. The differences in vibrational selection rules for surface Raman and infrared spectroscopy are discussed for the case of azide adsorbed on silver, and used to distinguish between "flat" and "end-on" surface orientations. Vibrational band intensity-coverage relationships are briefly considered for some other systems that are unlikely to involve coverage-induced reorientation. 

In addition to the Raman selection rules described above there are surface selection rules that apply for SERS because the process occurs close to metal surfaces [40—42]. The SERS surface selection rule predicts that the vibrational bands that have contributions from the Raman polarizability tensor component where z is the surface normal, will be most intense with weaker contributions from vibrational bands which have contributions from and o. This is essentially because tlic electric field of the exciting hght is enhanced in the direction of the surface normal (Figure 6.2). The surface selection rule for Raman spectroscopy is more complex than that for infrared spectroscopy. Modes with the bond axis paraUel to... 

There have been a number of reports of analogous surface selection rules for Raman spectra [27-29]. However, for SERS, the situation is complicated by the essential roughness of the metal surface and the mixture of enhancement mechanisms, in addition to the facts that the Raman effect depends upon the molecular polarizability tensor and the excitation frequencies are typically high enough to reduce the metal conductivity to levels where finite parallel, as well as perpendicular, electric vectors are established at the surface. An excellent recent review of this subject has been written by Creighton [30]. Suffice it to say here that surface selection rules evidently do exist for Raman spectroscopy but they are more complicated than the rule for infrared and EELS. 

A particularly salient example of the complementarity of normal Raman spectroscopy with other techniques is expressed in a recent publication of the combined data from infrared, normal Raman, SERS, SEHRS, and theoretical predictions for one molecule. First, ab initio theoretical predictions were made for the vibrational characteristics of fran5-l,2-bis(4-pyridyl)ethylene (BPE) at the Hartree-Fock 6-31G level. When the spectra were collected, comparisons were made between the theoretical and experimental results as well as among the different spectra. Based on the known selection rules for each spectroscopy and the matching of wavenumber shifts to theoretical predictions, all vibrational bands were assigned. 

The selection rules for the SERS intensities indicate that since the local fields are highest normal to the surface, normal modes of the polymer involving changes in the molecular polarizability with components normal to the surface are subject to the greatest enhancements. Thus it is possible by knowing the molecular polarizability of functional components of a polymer to elucidate the surface orientation of the adsorbed molecules. 

These experimental results confirm and extend the interpretations given earlier. We refer the reader to Ref. [21] for further details and discussion of these results in the context of SERS and in relation to surface selection rules [45, 46]. 

According to the SERS selection rules, the spectral profile of the adsorbate is strongly dependent on the orientation of the main molecular axes with respect to the surface . Thus, SERS intensities would also provide valuable information about the molecular orientation that the adsorbate adopts once adsorbed on the metal surface. For this reason, symmetry assignments are central to the discussion of molecular orientation of adsorbed species on the surface of island or colloidal metal particles. If the molecules, i.e. N4 macrocycles, are oriented face-on the metal surface, with the N atoms face to the metal atoms, the C4 axis of the molecule and the normal to the surface are parallel thus, the most symmetric vibrational modes, that derive their intensity from the zz-component of the polarisability derivative tensor a z will be the most intense at the surface plasmon resonance frequency and to the red of that frequency. 

SERS of amino acids, polypeptides and proteins For a successful interpretation of the SERS spectra of proteins it is essential to have detailed information about the SERS behaviour of their monomeric units and simple models. It is reasonable to expect that the intensity of their SERS bands depends on the orientation of the protein molecule with respect to the surface. On the basis of a careful assignment of spectral bands and surface selection rules proposed by Moskovits [63, 80, 81], the orientation of amino acids adsorbed on the surface of colloidal silver was predicted [82-84]. 

The SERS selection rules can be used to infer molecular orientation. For a situation where only the EM mechanism is important, a set of SERS selection rules can be derived. The expression for the effective polarizability when an enhanced local electric field of a rough surface bathes the adsorbed molecule is given by equation 1, below (4) ... 

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