The interaction of hydrogen atoms and ions with hydrocarbon (C:H) films plays an important role for the understanding of the growth and erosion of C:H films by plasma enhanced chemical vapor deposition. Elementary steps of the erosion mechanisms were recently identified in dedicated UHV experiments on ion beam deposited films. Furthermore, the interaction of hydrogen plasmas with C:H films was studied by ellipsometry in the visible spectral range. The interaction of H atoms as well as ions leads to a modified layer with a few nanometer thickness at the plasma facing surface of the film. We applied infrared spectroscopy (IRS) to investigate this modified layer created at the surface of C:H films with a thickness of only 10 nm, deposited on an optical cavity substrate. For the interpretation and quantification of the IRS data, a formalism is presented. It allows the calculation of the imaginary part kappa of the refractive index n of the thin film samples from the measured IR spectra and includes multiple coherent reflections in the film itself, multiple incoherent reflections in the substrate, and takes into account the Kramers-Kronig relation between the real and imaginary part of the refractive index.