Incidence angle dependence of the enhancement factor in attenuated total reflection surface enhanced infrared absorption spectroscopy studied by numerical solution of the vectorial Maxwell equations.

The finite element method (FEM) was used to solve the time-harmonic Maxwell equations in a study of the effect of the incidence angle of infrared light on the surface enhancement caused by colloidal gold particles in attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS). The spectral enhancement factor was obtained from computations of absorbance from a thin organic layer in the presence and absence of the metal nanostructure. For computations of an isolated particle the enhancement factor is high around the critical angle and decreases with an increase in incidence angle. This trend was also observed in experiments performed with gold particles immobilised on a silane modified silicon ATR crystal. Computations where gold particles are touching each other show low enhancement factors around the critical angle and an increase with increasing incidence angle. These two opposing trends are analysed based on the electric field distribution around the particle.