Calorimetric measurements of the energetics of Pb adsorption and adhesion to Mo(100)

The heat of adsorption of Pb on Mo(100) at 300 K has been measured calorimetrically using a single-crystal adsorption calorimeter and a pulsed atomic beam of Pb vapor. The heat of adsorption decreases with coverage in the first monolayer as ${(q}_0-q_1\theta -q_2{\theta }^2),$ where $q_0=335.4\mathrm{}\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l},$ $q_1=65.0\mathrm{}\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l},$ and $q_2=69.8\mathrm{}\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l},$ indicating a heat of adsorption for an isolated Pb adatom of 335.4 kJ/mol, but with a repulsive interaction between two Pb adatoms centered in nearest-neighbor sites of 27.9 kJ/mol, and a relaxation of this repulsion of 11.6 kJ/mol by an isolated pair if unperturbed by third neighbors (that would otherwise prevent the adatoms from moving slightly off site-center to achieve this relaxation). In the second monolayer, the adsorption heat is nearly constant at $\sim 192.7\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l},$ close to the enthalpy of sublimation of Pb. It decreases at $\sim 2\mathrm{ML}$ by $\sim 7\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l},$ which can be attributed to the conversion from layer-by-layer to three-dimensional island growth at $\sim 2\mathrm{ML}.$ The initial islands are tiny and therefore adsorbing Pb atoms more weakly (bonding to fewer nearest neighbors). This is supported by Auger electron spectroscopy measurements. As the islands grow in size and their surfaces get flatter, the number of neighbors increases until the heat of adsorption above $\sim 6\mathrm{ML}$ reaches a steady value of 191.5 kJ/mol, within 2% of the reported enthalpy of sublimation of Pb at 300 K (195.2 kJ/mol).