Semicrystalline polyethylene terephthalate (PET) films were irradiated with 35MeV/u Ar ions at room temperature. The ion beam induced surface modifications were studied by means of Fourier-transfer infrared spectroscopy in reflective mode as a function of the absorbed dose. Amorphization was evidenced by the decay of the bands related to the crystalline regions and the increase of the bands assigned to the amorphous regions in PET. Chain scissions were observed at the para position of benzene rings and C—O bond of ester. The benzene ring structures exhibited only small change during irradiation. These effects were found to depend on both ion fluence and the electronic energy loss near the sample surface. The creation of alkyne groups was found only at the absorbed dose higher than 5.0MGy

and its concentration increase with the increased of the absorbed dose. The results were qualitatively discussed.