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A series of novel Raman bands has been observed in the surface-enhanced Raman scattering (SERS) spectra of self assembled monolayers (SAMs) of alkanethiols on Au and Ag surfaces. The novel Raman bands display a resonance effect, observed with excitation in the 630 to 710 nm range. The bands exhibit a chain length dependence at room temperature, occurring in SAMs with n>9 methylene groups. At liquid nitrogen temperatures, SAMs with /; > 6 display the bands, indicating a dependence on monolayer structure. Alkanethiol isotopic labeling studies involving substitutions of '"'C and '*S reveal that the novel bands are not likely of a vibrational origin. In addition, SAMs of (CH3-(CH2)9-Se)2 on .Au yield identical spectra (within ± I cm* ) as the alkanethiol SAMs. Electrode potential studies of SERS have shovm that charge transfer effects are not the source of the bands. Impurities on the SERS elearode surfaces do aot appear to be source of the novel bands. INTRODUCTION The self-assembly of molecules to form monolayers, especially alkanethiols, on Au and .Ag metal surfaces has been extensively studied, in the past several years.'"* The relative ease of alkanethiol monolayer preparation, their well-ordered structure and their general ruggedness have all contributed to their study. The affinity of the thiol headgroup for a

Raman characterization of alkanethiols and the application of alkanethiols to electroreflectance spectroscopy