NEAR-INFRARED SURFACE-ENHANCED RAMAN-SCATTERING CAN DETECT SINGLE MOLECULES AND OBSERVE HOT VIBRATIONAL TRANSITIONS

Surface-enhanced Raman scattering (SERS) at an extremely high enhancem ent level opens up interesting and new spectroscopic possibilities. Th e effect combines the sensitivity of fluorescence spectroscopy with th e high structural information content of Raman spectroscopy, and can b e used for single molecule detection and identification. This paper re ports single molecule detection and identification of 'non-absorbing' molecules in colloidal silver solutions using near-infrared excited su rface-enhanced Stokes and anti-Stokes Raman scattering. SERS enhanceme nt factors of the order of 10(14)-10(15) or, in other words, effective Raman cross-sections between 10(-16) and 10(-15) cm(2)/molecule resul t in a significant transfer of ground state population to the first ex cited vibrational state due to the strong Raman process. This allows t he observation of v = 1 to v = 2 ('hot') vibrational transitions in SE RS additionally to v = 0 to v = 1 transitions 'normally' probed in a R aman experiment. (C) 1998 John Wiley & Sons, Ltd.