ENHANCEMENT FACTOR OF SURFACE-ENHANCED RAMAN-SCATTERING ON SILVER AND GOLD SURFACES UPON NEAR-INFRARED EXCITATION - INDICATION OF AN UNUSUAL STRONG CONTRIBUTION OF THE CHEMICAL EFFECT

Surface-enhanced Raman (SER) spectra of the xanthine dyes rhodamine 6G (R6G) and oxazine 1 (OX1) were measured with near-infrared excitation employing Fourier transform Raman spectroscopy. Both dyes exhibit strong SER signals on Ag and Au surfaces. The enhancement factors were evaluated based on the determination of the number of adsorbed molecules. In the absence of additional electrolyte, the enhancement factors for R6G on Ag and Au sols are 1.3 x 10(4) and 1.6 x 10(4), respectively. This enhancement is operative for all adsorption sites and is ascribed to the classical electromagnetic mechanism. On activation of the Ag sol by chloride ions, the enhancement factors for R6G and OX1 are 4.3 x 10(7) and 7.6 x 10(5), respectively. This increased enhancement which occurs at a small number of specific adsorption sites is ascribed to the charge-transfer mechanism. For R6G this enhancement mechanism is stronger by a factor of 47 than for visible excitation. The electrolyte-induced activation is not possible for Au sols or precipitates of Ag or Au on filter-papers. These metal-coated filter-papers may provide appropriate SER-active surfaces for analytical applications of the SER effect in combination with Fourier transform Raman spectroscopy.