Infrared reflectance spectroscopy on thin films: Interference effects

Laboratory simulations of processes on astronomical surfaces that use infrared reflectance spectroscopy of thin films to analyze their composition and structure often ignore important optical interference effects which often lead to erroneous measurements of absorption band strengths and give an apparent dependence of this quantity on film thickness, index of refraction and wavelength. We demonstrate these interference effects experimentally and show that the optical depths of several absorption bands of thin water ice films on a gold mirror are not proportional to film thickness. We describe the method to calculate accurately band strengths from measured absorbance spectra using the Fresnel equations for two different experimental cases, and propose a way to remove interference effects by performing measurements with P-polarized light incident at Brewster's angle.