Nondestructive Laser Method for Measuring Charge Profiles in Irradiated Polymer Films

Relative charge-density distributions in the thickness direction of thin (>10¿m) polymer films are determined directly and accurately with a new laser-induced pressure-pulse (LIPP) method: A pressure pulse is generated in the sample by applying a short (70ps) and energetic (1-11mJ) light pulse from a Nd:YAG laser to a specially coated surface of the sample. Stress effects within this surface layer and possibly the recoil caused by ablation of some target material generate the desired pressure pulse of less than 1ns duration which propagates through the sample. From the electrode currents, the charge distribution can be evaluated. The new method is described in detail and some results for 20 to 50¿m thick films of Teflon FEP, Mylar PETP, and Kynar PVDF are given.