Microcalorimetric measurements of the heat of absorption of Pb on well-defined oxides: MgO(100) and p(2*1)-oxide on Mo(100)

摘要

The energetics of Pb film grown at 300 K on two well-defined oxides [a MgO(100) thin film and a p(2xl)-oxide on Mo(100)] have been measured using single-crystal absorption microcalorimetry. The evolution of the film morphology was followed using Auger electron spectroscopy (AES). An initial heat of absorption of 103 kJ/mol is observed for the Pb/MgO(100) system. Auger results indicate that this is due to the formation of an -20 atom Pb island in the first pulse of Pb gas incident on the MgO(lOO) surface. This allows the extraction bf a Pb-MgO(loo) bond energy of -32::!:2 kJ/mol in the small two-dimensional particles formed ift the first pulse. As more Pb is deposited onto the MgO(loo) surface, the Pb forms three-dimensionalcislands. The integral of the heats of absorption up to high coverage indicates an adhesion energy of76.5::!:-20.uJ/cm2 for large 3D Pb particles to the MgO(100) substrate. This indicates a Pb-MgO(loo) bond energy of 49::!: 13 kJ/rnol at the 3D interface. Similar analyses of the Pb/p(2X 1)-oxide on Mo(100) surface give an initial heat of absorption of 146.2 kJ/mol and an adhesion energy of 82.5::!:20 .uJ/cm2 for thick Pb films. This indicates a bond energy of 52::!: 12 kJ/mol for a Pb atom to this surface at the interface of a thick Pb film. The metal's sticking probabilities as a function of coverage and the coverage-dependent changes in optical reflectivity were measured. For the Pb/MgO(100) system, an initial sticking probability of 0.70 at 300 K was observed, whereas for the Pb/p(2x 1)-oxide on Mo(100) surface unit sticking probability was observed. The relationship between these quantities and the adsorption energetics is discussed.