A Unified Approach to Surface-Enhanced Raman Spectroscopy

We present a unified expression for surface-enhanced Raman spectroscopy (SERS). The expression contains a product of three resonance denominators, representing the surface plasmon resonance, the metal−molecule charge-transfer resonance at the Fermi energy, and an allowed molecular resonance. This latter resonance is that from which intensity is borrowed for charge transfer, and when the molecular resonance is active it is responsible for surface-enhanced resonance Raman spectroscopy. We examine this expression in various limits, to explore the relative contribution or each resonance. First, we look at the situation in which only the surface plasmon resonance is active and examine the various contributions to…