The Electric Field Standing Wave Effect in Infrared Transmission Spectroscopy

When band ratios in infrared absorbance spectra of films are compared (which had been converted from transmittance spectra), it can be noted that even after background correction and removal of interference fringes these band ratios change with the thickness of the films. The main goal of this work is to show that this effect is a consequence of an electric field standing wave based on the coherent superposition of light waves in the film. We further investigate how transmittance and reflectance, as well as absorbance and the (from absorbance) regained index of absorption, depend on the thickness of the film and how these parameters influence the positions of bands. We compare the results with those for the incoherent case and the case where a single pass of light through the film without reflection loss is assumed. Catch the wave: Thin‐film infrared spectra are heavily influenced by an electric field standing wave effect. As one consequence, if the Beer–Lambert law is applied to convert transmission spectra into absorbance, peak ratios and the position of absorbance maxima show a dependence on the film thickness.