Nanospectroscopy highlights

Unlike surface catalytic reactions, desorption has been thought as a relatively simple process consisting of a series of statistically independent events randomly and uniformly occurring over the surface: in this picture adsorbates take off as soon as their thermal energy exceeds the binding energy. Following Irving Langmuir’s mean-field treatment, the rate of recombinative desorption of adsorbed particles is thought to be proportional to the square of the coverage. However, several measurements contradicted this model. As a matter of fact, temperature-programmed desorption (TPD), the standard method for determining desorption kinetics, hardly ever shows pure Langmuir behavior. The exponent m in the desorption rate law, referred to as desorption order, is often a fractional number rather than an integer (m = 1 or 2); further, the desorption peaks in TPD exhibit unexpected widths or

symmetries or are split into multiple peaks. In such cases, the desorption mechanism cannot be uniquely understood from TPD data. Instead, a detailed characterization of the complex microscopic mechanisms occurring into the adlayer is instead needed. In order to shed light on the microscopic origin of desorption, we have investigated oxygen on Ag(111) combining structure sensitive electron microscopy with TPD.

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Desorption kinetics from a surface derived from direct imaging of the adsorbate layer;
S. Günther, T.O. Menteş, M.A. Nino, A. Locatelli, S. Böcklein, and J. Wintterlin;
Nat. Comm. 5, 3853 (2014);
doi: 10.1038/ncomms4853.