The surface modification of polypropylene (PP) by monoenergetic argon ions and UV photons is evaluated in a particle beam experiment. Thereby, the polymer pre-treatment in a plasma process can be mimicked. The etching and chemical modification of the spin-coated PP thin films is monitored in real-time by in situ Fourier transform infrared spectroscopy (FTIR). It is shown that the initial exposure to the plasma ion source causes a modification of the film surface, which slows down the initially high etch rate. The separately measured UV-induced damage is more severe compared to the oxygen-containing polymer polyethylene terephthalate (PET).