Topographic micro scale and in-depth nano scale level modifications of polymeric materials play an important role in engineering their physical and chemical properties. Polyethylene terephthalate (PET) is an important class of semi-crystalline polymers used for gas separation properties. The gas diffusion and permeability parameters are directly related to the free volume fractions and the hole distributions. The controlled and precise ion beam irradiation can be used to induce surface and in-depth modifications in the properties of the polymers which help in modifying free volume holes and their distributions. In the present study, the investigation of free volume (nano scale level) and surface (micro scale level) properties of PET polymeric thin films after SHI treatment were employed by means of positron annihilation lifetime spectroscopy (PALS) and atomic force microscopy (AFM), respectively. The PET thin films were irradiated by 50 MeV lithium ions as a function of ion fluence. The value of hole radius (R) and intensity (I-3) of o-Ps were observed to be increased after ion beam treatment. The further analyses were employed to calculate the free volume and fractional free volume of holes from the obtained values of R and I-3. The AFM studies reveal the surface modifications of the irradiated polymer films. The structural, optical and chemical properties were investigated by X-ray diffraction (XRD), UV-visible (UV-vis) and Fourier transform infrared (FTIR) spectrophotometry. Different parameters such as crystallite size and band gap energy were calculated from the obtained data of XRD and UV-vis, respectively. (C) 2015 Elsevier B.V. All rights reserved.