Computation of surface-enhanced infrared absorption spectra of particles at a surface through the Finite Element Method

The finite element method (FEM) was used to solve the time-harmonic Maxwell equations to calculate full infrared (IR) absorption spectra of organic substances near particles adsorbed to a silicon/air interface mimicking a surface-enhanced infrared absorption spectroscopy (SEIRAS) experiment in internal reflection geometry. An enhancement factor attributed to the presence of the metal has been defined, and its dependence on the particle size and arrangement has been studied. The enhancement factor was found to increase with increasing particle size. Contributions of parts of the surface to the overall absorbance has been analyzed through the electric field patterns and by placing a probe material in the region to be analyzed. For isolated particles, the gap between particle and surface has a large contribution to the overall spectrum. Further, large contributions were found from the regions between touching particles. An experiment probing gold particles on a silicon surface using attenuated total internal...