Raman Scattering of Adsorbates

The purpose of this section is to provide some background to the theories by introducing theoretical expressions for Raman scattering intensities and outlining some of the more important features of the various theories classical electromagnetic enhancement (EM enhancement) and nonclassical contributions (chemical effects). 

The intensity of a Raman line corresponding to a transition between an initial state, i, and a final state, f, is given by  

The normal Raman scattering (NRS) regime is defined by the condition hfi L (E — Ej). Resonance Raman scattering (RRS) is defined by the condition hoL (Ee — Ej). In RRS the value of dcr/dQ corresponding to the by resonance enhanced value of can be 10 to 10 times greater than in the corresponding NRS case. 

In 1977 two groups, Jeanmaire and Van Duyne and Albrecht and Creighton showed that the Raman cross section for pyridine adsorbed onto electrochemically roughened silver electrodes was enhanced 10 times  

There is some experimental evidence to indicate that much of the enhancement is associated with surface roughness (local microstructures) in the range of 1 to 100 nm. At resonance with th microparticle modes the local electric field at the incident frequency (h L) becomes large near and on the particle surface. Furthermore, the re-radiation efficiency of Raman active molecules situated near the surface also becomes enhanced when the inelastically scattered frequency (hco,) is also in resonance with these microparticle modes. The Raman scattering intensity of the adsorbed molecule is then given by 

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Otto A 1991 Surface-enhanced Raman scattering of adsorbates J. Raman Spectrosc. 22 743-52... 

Tran [391,392] has reported ng detection of dyes on filter paper by SERS. Silver colloidal hydrosols stabilised by filter supports enhance the Raman scattering of adsorbed dyes. Typical detection limits are 500 pg (crystal violet), 7 ng (l,l,9-trimethyl-2,2 -cyanine perchlorate), 15 ng (3,3 -diethylthiacarbocyanine chloride) and 240 ng (methyl red) using a 3 mW He-Ne laser. 

Figure 20.3 Enhanced optical properties of gold nanoparticles resulting from the interaction of light with a gold nanoparticle including absorption, Mie scattering, fluorescence and surface nhanced Raman scattering of adsorbed molecules. Reprinted with permission from ref 3. Copyright 2007 Future Medicine.

Otto A (1991) Surface-enhanced Raman scattering of adsorbates. J Raman Spectrosc 22 743... 

The aim in the following presentation is to discuss the published results on Raman scattering of adsorbed dyes, from today s perspective, and see whether a clear and unambiguous picture can be reached. 

The metal substrate evidently affords a huge ( 10 and even as high as 10 [84, 85]) increase in the cross-section for Raman scattering of the adsorbate. There are two broad classes of mechanisms which are said to contribute to this enhancenient [, and Ml- The first is based on electromagnetic effects and the second on cheniicaT effects. Of these two classes the fomier is better understood and, for the most part, the specific mechanisms are agreed upon the latter is more complicated and is less well understood. SERS enhancenient can take place in either physisorbed or chemisorbed situations, with the chemisorbed case typically characterized by larger Raman frequency shifts from the bulk phase. 

W. H. Weber and G. W. Ford, Enhanced Raman scattering by adsorbates including the nonlocal response of the metal and excitation of nonradiative modes, Phys. Rev. Lett. 44, 1774-1777 (1980). 

Nikoobakht B, Wang JP, El-Sayed MA (2002) Surface-enhanced Raman scattering of molecules adsorbed on gold nanorods off-sruface plasmon resonance condition. Chem Phys Lett 366 17-23... 

SERS involves the dramatic enhancement of Raman scattering of the molecules adsorbed on nanostructured metal surface [71]. It is currently widely accepted that... 

Pang YS, Hwang HJ, Kim MS (1998) Reversible temperature dependence in surface-enhanced Raman scattering of 1-propanethiol adsorbed on a silver island film. J Phys Oiem B 102 7203... 

Taniguchi, L. Umekita, K., and Yasukouchi, K. (1986) Surface-enhanced Raman scattering of nicotinamide adenine dinucleotide (NAD ) adsorbed on silver and gold electrodes. 

The estimate of the degree of enhancement in every one of our experiments is of prime importance for understanding SERS. First, the enhancement factors vary from system to system and that may reflect different contributions of the various possible mechanisms. Second, the sensitivity of the Raman systems has improved thus, detection of a signal does not necessarily mean that an enhancement of four to six orders of magnitude occurs. Nowadays one can detect signals which are enhanced by a factor of only 100 to 1000 or less. The Raman scattering of dyes adsorbed on surfaces can be detected even without any enhancement at all. 

Large enhancements are measured for the Raman scattering of molecules adsorbed on (silver) metal surfaces common enhancement factors are 10" -10. ... 

There are claims that also aluminum produces enhancements. Liao and Stern " saw Raman scattering of p-nitrobenzoic acid adsorbed on oxide-covered ( 3-nm thickness) aluminum ellipsoids, which was 500-fold weaker than exhibited on silver. The intensities on aluminum increased by a factor of 3 changing the exciting energy from 2.2 to 2.7 eV, though the optical parameters of aluminum are rather constant in this region. Lopez-Rios et also reported surface Raman spectra from (perhaps) molecular oxygen and carbon oxides adsorbed on aluminum in UHV conditions. No SERS of pyridine was seen. 

C. A. Murray, D. L. Allara, and M. Rhinewine, Phys. Rev. Lett. 46 (1981) 57. A. Bachackashvilli, Z. Priel, S. Efrima, and B. Katz, Raman scattering of methyl-red adsorbed on silver colloids, unpublished paper. 

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