Surface Enhanced and Unenhanced Raman Scattering of Alkanethiols Adsorbed on Silver and Gold Surfaces

Raman spectroscopy has been demonstrated to be a powerful tool with which to probe the orientation of bonding of molecules on metal surfaces. Surface enhanced Raman scattering (SERS) has been successfully used for this purpose, but suffers from the requirement for considerable surface roughness in order for the ehancement to be realized. A more desirable approach is the acquisition of Raman spectra of molecules on smooth metal surfaces. This is accomplished in our laboratory with charge-coupled device (CCD) detection which provides adequate sensitivity for the observation of monolayers of organic molecules. This detector is coupled to a triple spectrometer and used with Ar+-pumped dye laser excitation between ca. 600 and 720 nm. Excitation in the red region of the spectrum is essential for enhancement on Au surfaces. The high quantum efficiencies of extended-red CCDs in this region provide excellent sensitivity for these measurements.