Optically absorbing layers on ion beam modified polymers: A study of their evolution and properties

Nothwithstanding the well known drastic changes wrought upon the optical properties of ion beam irradiated polymers, no systematic search for the corresponding physical mechanism responsible can be found in the literature. Accordingly, a study was made of the depth profiles of optical absorption in a number of irradiated polymers, using radiation extending in wavelength from the ultraviolet, through the visible, and into the infrared. Work concentrated on the mass and dose (fluence) dependence of absorption for noble gas ions. It is shown that for visible light the shape of the depth absorption profile closely follows the distribution with depth of nuclear energy transfer processes, calculated in the normal fashion. This suggests that the optical absorption itself is directly due to polymer degradation by direct knock-on of atoms from the chains. A pronounced maximum in the dependence of the integrated absorption of visible light on projectile mass occurs for neon ions. Furthermore a similar mass dependence is demonstrated for the "kinematic factor" in nuclear collisions with carbon, thus supporting the direct knock-on hypothesis. Finally, an annealing of the irradiation induced polymer damage - a blackening becoming lighter - is also noted. It is ascribed to a specific photochemical reaction.