SERS model, charge transfer

Although electromagnetic enhancement can explain important features of the SERS effect, it does not take into account in any way the chemical structure of the analyte species. Therefore, chemical enhancement models take into account the structural properties of the analyte and it is thought to operate independently from electromagnetic enhancement. Chemical enhancement can result from charge-transfer or bond formation between the metal and the analyte molecule which can result in an increase in the polarizability, ot, of the molecule. Therefore, a surface complex between the analyte and the metal must form before chemical enhancement can occur. 

They suggested that the same model is valid for SERS, where due to the high polarizability of the metal electrons, large enhancements are expected. They also predict that the molecule-metal vibrational mode will be exceptionally intense, as it strongly affects the degree of charge transfer. 

Early attempts of applying this approach to molecular plasmonics (namely to SERS) were done already in the 80s and 90s [101-107]. They were however limited to metal clusters composed of a few atoms. More recently [108,109], the power of this approach has been fully disclosed by treating metal nanoparticle (Ag2o) that has excitation with a qualitative character of plasmons. In fact, a study of chemical effects was possible with this approach, that is precluded to the simpler models discussed in the previous sections. For example, the importance of charge transfer could be qualitatively investigated (despite some limitations of the underl dng ab initio methods), and involved orbitals could be identified (see Fig. 5.11). 

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