Thickness-dependent intensities and positions of absorption lines in infrared reflection-absorption spectra of thin films on reflective substrates at near-normal incidence are investigated. Two types of absorption bands in a polymer, the weak and the strong, were examined. Their optical properties were determined by the dielectric response function. The optical path of the beam was described by the coherent sum of all successively reflected beams. The thickness-dependent properties of absorption bands were examined in three typical thickness regions. At small thickness, the peak intensity oscillates around the mean value defined by the simple internal absorptance of the beam crossing the double layer. For medium thickness, the peak position swings around the original frequency and its intensity oscillations move above the simple internal absorptance. In layers within the high-thickness region, optical distortions cause large changes in line shape due to approaching the bulk reflectance. A simple analytical interpretation is possible only within the low-thickness region. The width of these thickness regions depends on the absorptivity of the considered band; for strong bands they are considerably narrower than for weak bands. The theoretically predicted effects compare well with those measured in RAS spectra of variously thick silicon resin layers on aluminium substrates.

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