Combined computational quantum chemistry and classical electrodynamics approach for surface enhanced infrared...

Surface enhanced spectroscopy, which enhances the signal intensity of molecules on a surface, facilitates the study of molecular properties, even down to a single-molecule level if a scanning probe is used. To realize the full potential of surface enhanced spectroscopy, a clear theoretical understanding is indispensable. However, quantum chemical calculations for surface enhanced spectroscopy are not simple because of the violation of the widely used dipole approximation. The spatial structure of electric near-field in the close proximity of a surface strongly depends on the geometry of the metal nanostructure as well as on the incident wavelength. Therefore, in principle, a universal model for electric near-field cannot exist. To address this issue, we have developed a generalized light-matter interaction model from first-principles quantum chemical calculations by using the multipolar Hamiltonian, in which the spatial structure of the electric field is fully considered. Here, we incorporate computational electrodynamics for surface enhanced infrared (IR) absorption spectroscopy in the model, where electric near-field around a Ag ellipsoid is obtained and used for IR calculations. Furthermore, we have devised a method to successfully reproduce the peak selectivity observed experimentally.