IR absorption enhancement for ionized p-nitrobenzoic acid on silver island films epitaxially grown on the 7 × 7 reconstructed Si(111) surface

An enhanced infrared absorption is reported for p-nitrobenzoic acid (PNBA) adsorbed on molecular-beam-epitaxially grown silver films at room temperature on 7×7 reconstructed Si(111) substrates. Reflection high-energy electron diffraction reveals the formation of Ag(111) islands even for 82-monolayer-thick silver deposition on the substrate. The NO₂ symmetric stretch of the surface-ionized species (PNBA^-) is a maximum in intensity at a thickness of 64 monolayers and is one order of magnitude stronger than the same mode of PNBA deposited at a thickness of 5 nm on a Si wafer. Based on the change in film morphology and in light transmittance with silver film thickness, this absorption enhancement is ascribed to the excitation of transverse collective electron resonance of the silver islands. The thickness dependence of the enhancement and of the film morphology will be compared with those observed for silver films deposited on oxidized Si(111) substrates. PACS 61.14.Hg; 68.37.Hk; 78.30.-J