Gap mode Raman spectroscopy under attenuated total reflection geometry

Gap mode Raman spectroscopy, which exploits a coupling of electrical dipoles in metal nanoparticles with their image dipoles induced in the neighboring substrates, was successfully coupled with a propagating surface plasmon (PSP) under attenuated total reflection (ATR) geometry. We obtained sufficiently large (10(7)-10(9)) enhancement factors for Raman scattering intensity from a thiophenol (TP) monolayer sandwiched by Ag films on prism and AgNPs under ATR geometry, which are significantly larger by a factor of 10-100 than those obtained under external geometry. Furthermore, much larger enhancement factors were obtained for smaller surface coverage of AgNPs owing to efficient excitation of a PSP.