THE ELECTROMAGNETIC THEORY OF SURFACE ENHANCED SPECTROSCOPY

Surface enhanced spectroscopy (SES) was born in 1 974 with the measure­ ment of Raman spectra of molecules adsorbed on a roughened electrode surface (la-e). Given the smallness of the Raman cross section, the detection of a Raman signal should have generated some excitement. This did not happen, but was delayed until 1977 when Jeanmaire & van Duyne (2) and Albrecht & Creighton (3) showed that the rough silver surface enhances the Raman cross section by a factor ranging between 104 and 106. This started a great outpouring (4a-i) of theoretical and experimental work, whose central theme is to understand how the presence of a solid surface modifies the spectroscopic and photochemical properties of a molecule located nearby. We divide the effects of the surface in two categories: electromagnetic, which can be described by solving Maxwell's equations, and chemical, which belong to quantum chemistry. The enhancement of the local laser field due to the polarization of the surface is an example of an elec­ tromagnetic effect. The appearance of a new excited state caused by chemisorption, leading to an enhancement of the Raman cross section through resonance Raman scattering (which would not be expected on the basis of the gas phase properties of the molecule), is an example of a chemical effect. While there is no doubt that chemisorption modifies the optical response of adsorbed molecules, the magnitude of the modification is still a subject of controversy. We don't know whether we should expect large modifications for all molecules, or for a small class (e.g. those with 11: orbitals); or whether