SIMULATION OF HYDROGEN THERMAL-DESORPTION FROM TRANSITION-METALS USING A MULTIPLE-SITE HOPPING MODEL

A model to simulate thermal desorption is described. The model is appl icable to systems where adsorbate-adsorbate lateral interactions are n egligible and fast recombination occurs, as in the case of hydrogen de sorption from transition metal surfaces. Thus, our model uses simple r ules to simulate desorption. If two atoms are sitting in neighboring s ites, then there is a high probability for the desorption of both to h appen. If an atom does not have a neighbor, it will move to adjacent s ites, in a random walk, until it finds a neighbor to recombine. The de sorption rate plotted as a function of the logarithm of the number of random steps results in perfectly shaped Gaussian curves. Dimensionles s curves of the rate versus the logarithm of the number of random step s were obtained for different coverages from 0.1 to 1.0 using this met hod. These curves are in perfect agreement with calculated curves for second-order desorption arising from the corresponding Polanyi-Wigner differential equation.