Depolarization effects due to the interaction between adsorbed dipoles on stepped surfaces

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Abstract
A generalization of the Helmholtz equation for the interpretation of adsorbate-induced work-function changes (WFC’s) on stepped surfaces, which accounts for mutual depolarization effects on the adlayer, is studied and discussed. The proposed equation, in connection with concentration profiles obtained through Monte Carlo simulations, allows us to study simultaneously depolarization and adsorbate distribution effects on the WFC. The limitations of WFC versus coverage measurements, performed at constant temperature on stepped and polycrystalline substrata, to obtain information about the surface distribution and the polarizability of the adsorbate are also discussed. The dependence of the electrical depolarization fields and the dipolar interaction energy on both the polarizability and the distance to the step have also been studied. It is found that under certain circumstances specified in the text, the model predicts an unexpected phenomenon called depolarization-induced dipole inversion, consisting in the change of the direction of some dipoles due to the dipolar interaction. This phenomenon would cause an oscillatory behavior of the curves of WFC versus coverage, provided that the nucleation of the adsorbate at the steps proceeds in a row-by-row mode.