Influence of coverage on adsorbate diffusion measurements at electrode surfaces by in situ linear optical diffraction

In situ linear optical diffraction is a new method for studies of surface mass transport in electrochemical environments that is based on the equilibration of coverage gratings in an adlayer on the electrode surface. We, here, discuss the temporal evolution of the diffraction intensity on the basis of experimental data for sulfur adsorbates on Pt(111) electrodes in 0.1M H2SO4 and simulations of the time-dependent diffusion profiles. At low and medium sulfur coverage, the decay of the signal exhibits two time scales, which can be explained by the influence of coverage-dependent diffusion rates on the evolution of gratings with large coverage modulation. At high coverage, a further ultra-slow decay process or even a complete termination of the decay is observed, which we attribute to the presence of high-density, ordered, adlayer phases with low sulfur mobility. These results provide insight into the approaches required for extracting quantitative surface transport rates from linear optical diffraction measurements.