Thermodynamics of surface clusters—direct observation of Re2 on W(211)

Abstract

Direct observations in the field ion microscope make it possible to determine the ratio of bound to dissociated dimers at equilibrium on a crystal surface. In principle, the thermodynamics of dissociation are accessible from the temperature dependence of this ratio. To establish the feasibility of such measurements, the kinetics governing changes in the dimer probabilities are worked out. This is done for isothermal processes, and for conditions typical of the quench from the temperature at which the equilibrium distribution is established to the temperature at which observations are made in the field ion microscope. Techniques are developed to correct for distortions of the equilibrium distribution during the quench. These rely upon measurements of the ratio of dimers in configuration 1 and 0 at low temperatures, combined with observations of P D , the fraction dissociated at high temperatures. Experiments conforming to this scheme have been carried out for rhenium dimers on W(211). They yield internal energies of −3.7±1.1 kcal/mole for rhenium dimers in configuration 1, and −2.3±1.2 kcal/mole for dimers in configuration 0, measured relative to the energy of dissociated pairs. Our results suggest important contributions from indirect interactions.