Structure and Surface Binding: N and CO on Tungsten

The energetics of the interactions of N2 and CO with initially clean tungsten surfaces are analyzed to establish the dependence of chemisorption upon surface structure. For N2, calorimetric measurements yield a heat of adsorption of ∼90 kcal mole—1, constant over the range of coverages accessible at T=300°K. This suggests that the equilibrium binding sites for nitrogen are energetically equivalent. Desorption obeys bimolecular kinetics, with an activation energy of ∼81 kcal mole—1, unchanging to the highest surface concentrations (n∼250×1012 molecules cm—2). This constancy confirms the energetic equivalence of the surface sites filled by nitrogen, and demonstrates the presence of adatoms without lateral interactions. Alternative proposals for the occupation of binding sites of different strengths are examined and rejected as caused by experimental error. In contrast, evolution of CO takes place in distinct steps, with activation energies of 53, 75, and ∼100 kcal mole—1 for β material. Stepwise desorption stems from the arrangement of CO binding sites. These differ in strength, are widely separated from one another, and presumably consist of planes with different crystallographic orientation. Despite the constant heat of adsorption, the presence of an activation barrier to surface diffusion, amounting to ∼35 kcal mole—1 on the approaches to the (100), demonstrates that binding of nitrogen atoms does depend on surface structure. In the regions between equilibrium sites, a nitrogen adatom is less firmly bound. On the (110) planes, in which each tungsten atom has six nearest neighbors, the binding energy of a nitrogen adatom is lowered to such an extent that at room temperature these areas should be bare. This is confirmed by direct observation in the field ion microscope. Such ``structural exclusion'' arises from the high dissociation energy of N2, which precludes occupation of sites accessible to other, less strongly bonded molecules. For nitrogen atoms only a narrow band in the total spectrum of binding energies is available. Structural exclusion is also shown to be in accord with the following unique properties of nitrogen layers: (1) The limited concentration (∼40% of the total number of surface sites) achievable at room temperature; (2) The presence of an additional and heavily populated binding state (γ) at T<200°K; (3) The absence of interactions between adatoms.