Diffusion of ethylene and xenon in thin pyrazine layers

Abstract

The rate of diffusion of C{sub 2}H{sub 4} and Xe through thin pyrazine films condensed on coldly deposited silver films was studied by following the time evolution of the surface-enhanced Raman intensity of the diffusant, as a measure of its accumulation at the silver-pyrazine interface. The form of the intensity versus time curve was reproduced well by solving the diffusion equation with the appropriate boundary conditions and including, in addition, contributions from depolarization effects that become significant at high diffusant surface concentration. The temperature dependence of the diffusion coefficient of C{sub 2}H{sub 4} through the pyrazine film was found to be of the form D = 6 {times} 10{sup {minus}11} exp(-0.7 kcal/RT) cm{sup 2}/s, indicating that the pyrazine films deposited on cold surfaces (40-60 K) have an amorphous structure. In addition to diffusion data. SERS spectra were measured at various silver film deposition temperatures for the mixed C{sub 2}H{sub 4}-pyrazine layer adsorbed on the silver surface after C{sub 2}H{sub 4} diffusion through the pyrazine film occurred. The spectra showed evidence for two different types of SERS-active sites on these rough Ag films, with the relative numbers of each type of site depending on the surface preparation temperature of the Agmore » film. Since the diffusion coefficient was apparently independent of the Ag surface preparation temperature, these studies also indicate that neither of these SERS-active sites are located at the bottom of long narrow pores as postulated by the pore model of Albano et al. Comparison between diffusion coefficients determined for C{sub 2}H{sub 4} and Xe shows that the diffusion coefficient follows a M{sup {minus}{1/2}} mass dependence as predicted by simple kinetic theory treatments of diffusion.« less