Surface Raman charge transfer enhancement

Grochala, Kudelski, and Bukowska [228] have described the anion-induced charge-transfer enhancement in S E RS and S E RRS spectra of rhodamine 6G on Ag electrode as a function of the electrode potential, upon addition of chloride and citrate anions. In a very recent paper, Brolo et al. [229] have discussed the ratio of the surface-enhanced anti-Stokes scattering to the surface-enhanced Stokes-Raman... 

Recently, Ueba presented a calculation of the Raman scattering polarizability of an adsorbed molecule on a metal surface, where charge transfer excitation participates as an intermediate state of the Raman process. A typical estimation of pyridine chemisorbed on Ag through the N lone pair electron shows an enhancement of about 1(P. This seems to indicate that the charge transfer mechanism (CT) operates in conjtmction with the EM effect. 

The nature of the mechanism that produces SERS is still the subject of debate. Two main mechanisms of enhancement are now most commonly proposed. These are electromagnetic enhancement and charge transfer or chemical enhancement. Electromagnetic enhancement does not require a chemical bond between the adsorbate and the metal surface. It arises from an interaction between surface plasmons on the metal surface and the adsorbed molecule. Chemical or charge transfer enhancement requires a specific bond between the adsorbate and the metal plus energy transfer between the metal and the adsorbate during the Raman scattering process. There is evidence for both mechanisms. The predominant view appears to be that both may occur. 

For a Raman resonance when od — (o u the denominator of the polarizability tensor component is of the order llFj. It follows that the surface Raman polarizability component may be reduced by approximately a factor of eleven over that in solution. This would lead to a Gserrs factor (Eq. 76) which would be circa ten times smaller than would be predicted if the polarizability components on the surface and in solution are equal. It should be pointed out, too, that a similar metal damping process would also take place for the charge transfer enhancement process. 

The relative importance of the two mechanisms - the non-local electromagnetic (EM) theory and the local charge transfer (CT) theory - remains a source of considerable discussion. It is generally considered that large-scale rough surfaces, e.g. gratings, islands, metallic spheres etc., favour the EM theory. In contrast, the CT mechanism requires chemisorption of the adsorbate at special atomic scale (e.g. adatom) sites on the metal surface, resulting in a metal/adsorbate CT complex. In addition, considerably enhanced Raman spectra have been obtained from surfaces prepared in such a way as to deliberately exclude one or the other mechanism. 

With the aid of a bi-functionalized reagent (terminated with pyrenyl unit at one end and thiol group at the other end), gold nanoparticles were self-assembled onto the surface of solubilized carbon nanotubes [147], Raman spectrum of the gold nanoparticle bearing CNTs is enhanced possibly due to charge transfer interactions between nanotubes and gold nanoparticles. 

Figure 6.1-14 To illustrate the molecular (o) enhancement of SERS due to resonance Raman scattering for excitation into a charge transfer band of the adsorbate-metal-surface-complex the filled 7t and empty TC orbital levels of pyridine and the metallic density of states of copper are shown on the left- and right-hand side, respectively for details see text (Creighton, 1986).

Shibamoto K, Katayama K, Sawada T (2007) Ultrafast charge transfer in surface-enhanced Raman scattering (SERS) processes using transient reflecting grating (TRG) spectroscopy. Chem Phys Lett 433 385-389... 

Reipa, V., Gaigalas, A.K., Edwards, J.J., and Vilker, V.L (1995) Surface-enhanced Raman spectroscopy (SERS) evidence of charge transfer between putidaredoxin and a silver electrode. Journal of Eiectroanaiytical Chemistry, 395, 299-303. 

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