Ultra thin polyimide layers have been prepared by the Langmuir Blodgett method and bombarded with a beam of 170 keV Ne+. Optical absorption and specular reflectance studies have been performed in the spectral domain 2.5–6.5 eV. The energy loss function is deduced and shows the development of a new absorption band at 3 eV, which accounts for the shift of the absorption edge towards the visible in thick films. An effective medium calculation shows that this resonance can be attributed to graphitic nanoparticles dispersed in the polymeric matrix. The development of these conducting domains accounts for the transition to a conducting regime at high irradiation fluence.