Surface enhanced Raman (SER) spectra of ortho, meta, and resonance excitation of surface plasmon polariton (SPP) and para formyl pyridines in silver sol are compared with their normal is termed classical electromagnetic effect. In addition, strong Raman spectra in bulk and aqueous solution. Significant enhancecoupling between the conduction electrons of the metal and ment of Raman bands has been observed. The SER spectra suggest the adsorbed molecule (8, 9) may result in charge transfer an almost normal orientation of the adsorbate molecular planes between them when a monomolecular layer is formed on to the metal surface and also reveal that ortho and para isomers the metal surface, and this may contribute to enhancement.
exist in a hydrate form in the surface adsorbed state. The meta
There may be one or more s donors (usually nitrogen or isomer, however, retains its original nicotinaldehyde form. On adoxygen) and also a p donor system (such as an aromatic dition of the formyl pyridines (FP), the n max of the bands that appear on the longer wavelength side of the electronic absorption ring) in the scattering molecule, in which case the possibility band of stable, neat silver sol show a parallel relationship with of surface adsorption increases (10). Further, the position the Hammett s (s H ) values of the isomers. Raman excitation of the substituent group affects the electronic structure of wavelength dependence suggests that resonance to the aggregation the molecule and hence the adsorption process on the metal band contributes significantly to SER spectra. The concentration surface.
dependence of the enhancement ratio shows a maximum, sug-
In this report we present the surface enhanced Raman gesting monolayer formation at a specific concentration for each (SER) spectra of three isomers of formyl pyridines (FP) isomer. This ''first-layer effect'' and also the excitation wavelength adsorbed onto silver sol under identical experimental condidependence of SER spectra imply a charge transfer contribution tions and discuss the results in terms of surface enhancement, to enhancement.