MEASURING DIFFUSION IN THIN-FILMS BY DISSOCIATIVE ELECTRON-ATTACHMENT- O(2) IN KR

We present data that indicate that for O2 deposited onto a Kr film, th e desorption of O- following the dissociative electron attachment (DEA ) of 6 eV electrons can occur only if the molecule resides at the film surface. For submonolayer quantities of O2, the O- signal is a functi on of the O2 coverage and thus may be used as a probe of the O2 surfac e concentration. With this method, O2 molecules originally buried with in a multilayer Kr film are observed to diffuse to the surface for var ious overlayer thicknesses deposited at 21 K. These experimental resul ts are compared with calculations based on the solution of the Markovi an master equation of the diffusion process. The analysis yields a val ue of 3 X 10(-20) m2/s for the diffusion coefficient of O2 through Kr to the film surface. It is orders of magnitude higher than predicted f rom various models based on O2 diffusion strictly via vacancies. These experiments indicate the possibility of using DEA as a probe to measu re the concentration of molecular surface impurities on molecular soli ds and dielectrics.