Studies of surface-enhanced Raman scattering from pyridine adsorbed on Ag surfaces in ultrahigh vacuum show a strong dependence on both surface roughness and pyridine coverage. The observation of enhancement (${10}^4$) for physisorbed pyridine multilayers in addition to the first molecular layer implies that the enhanced effect is electromagnetic rather than chemical in origin.

• Received 3 March 1980

DOI:https://doi.org/10.1103/PhysRevLett.44.1770