Surface-enhanced Raman scattering (SERS) at an extremely high enhancement level opens up interesting and new spectroscopic possibilities. The effect combines the sensitivity of fluorescence spectroscopy with the high structural information content of Raman spectroscopy, and can be used for single molecule detection and identification. This paper reports single molecule detection and identification of ‘non-absorbing’ molecules in colloidal silver solutions using near-infrared excited surface-enhanced Stokes and anti-Stokes Raman scattering. SERS enhancement factors of the order of 1014–1015 or, in other words, effective Raman cross-sections between 10-16 and 10-15 cm2/molecule result in a significant transfer of ground state population to the first excited vibrational state due to the strong Raman process. This allows the observation of v=1 to v=2 (‘hot’) vibrational transitions in SERS additionally to v=0 to v=1 transitions ‘normally’ probed in a Raman experiment.
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Kneipp et al. (1998) studied this question.