| Abstract
| - The adsorption of Ca on the MgO(100) surface at 300 K has been studied using microcalorimetry,in combination with LEED, AES, ISS, work function, sticking probability measurements, and density functionaltheory (DFT) calculations. The MgO(100) thin films (∼4 nm thick) were grown epitaxially on a 1 μm thickMo(100) single-crystal. The sticking probability of Ca on MgO(100) at 300 K is unity. On the basis of AESand ISS measurements, it was determined that Ca grows mainly as 3D particles on the MgO(100) surfacewith a density of ∼1 × 1012 islands/cm2. Ca adsorbs initially at defect sites with a very high heat of adsorption(∼410 kJ/mol). DFT calculations attribute this high initial heat to Ca binding to kink sites (376 kJ/mol), stepsites (205 kJ/mol), and lower concentrations of stronger binding sites. The heat of adsorption decreasesrapidly with coverage, reaching a minimum of 162 kJ/mol at ∼0.3 ML, where Ca is mainly adding to small3D Ca clusters. Afterward, it increases to the value of bulk Ca heat of sublimation (178 kJ/mol) at ∼1.2ML, attributed to the increase in stability with increasing Ca particle size. A 1.0 eV decrease of the workfunction with Ca coverage from 0 to 0.3 ML indicates that Ca adsorbed at defects is cationic, in agreementwith calculations showing that Ca donates electron density to the MgO. Light ion sputtering of the MgO(100) surface generates point defects, but these do not change the heat of adsorption versus coverage,implying that they do not nucleate Ca particles. Oxygen vacancies are a likely candidate; DFT calculationsshow that F and F+ center vacancies bind Ca more weakly than terrace sites. More extensive sputteringcreates extended defects (such as steps and kinks) that adsorb Ca with heats of adsorption up to ∼400kJ/mol, similar to that at the intrinsic defect sites.
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