Classical Model of Surface Enhanced Infrared Absorption (SEIRA) Spectroscopy

The molecule–plasmon interaction is the key to the mechanisms of surface enhanced infrared absorption (SEIRA) and surface enhanced Raman scattering (SERS). Since plasmons are well described by Maxwell’s equations, one fundamental treatment involves the classical interpretation of infrared absorption and resonance Raman spectroscopies. We can understand the molecule–plasmon interaction using electromagnetic theory if the classical field effect on a transition dipole moment or transition polarizability is properly described. In previous work, we derived the Raman excitation profile of a model molecule using a classical driven spring attached to a charged mass with a perturbative force constant due to vibrational oscillations. In this study we generalize the interactions of plasmons with molecules by considering the N₂O asymmetric stretch SEIRA signal on a Dy doped CdO (CdO:Dy) film. This semiconductor has tunable plasmon dispersion curves throughout the near-and mid-infrared that can interact directly with vibrational absorption transitions. We have demonstrated this using the Kretschmann configuration with a CaF₂ prism and a MgO substrate. The model predicts the phase behavior of SEIRA. The calculated enhancement factor relative to an Au control is 6.2, in good agreement with the value of 6.8 ± 0.5 measured under the same conditions.