Adsorption of Potassium on Tungsten

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The adsorption of potassium on thermally annealed and field-evaporated tungsten surfaces was studied by field emission. The φ̄- vs-θ̄ curve has a minimum of 1.78 eV at θ̄=0.83, n̄=3.2×1014 atom/cm2 (which coincides with the minimum of φ110), and approaches φK at θ̄=2–3. Distinct minima also occur for other close-packed planes, like 211. At θ̄=1, n̄=3.9×1014 atom/cm2, φ̄=1.95 eV the emission pattern is indistinguishable from that of clean tungsten. The average charge per K atom, q̄, varies from 0.27 at low θ to 0.2 electron charges at θ̄=0.8. The value of q on the close-packed W planes at low coverage may be considerably higher. The activation energy of neutral desorption varied from 2.67 to 2.4 eV over the interval 0<θ̄<0.4 and fell rapidly at higher coverage, approaching the heat of sublimation of K, 1.0 eV, at θ̄=1.0. The activation energy of ionic desorption was measured over the interval 0<θ̄≤0.07, and indicates that ionic desorption occurs on regions of φ=5.3 eV. dφ/dθ̄ on these regions is very high, suggesting that the K-atom density there exceeds the average by a factor of 3 or more, and that adsorption on these regions may be nearly ionic at low θ̄. The activation energy of surface diffusion was measured quasidifferentially as a function of θ̄. Ediff and the pre-exponential term of the diffusion coefficient increased with θ̄ in the range 0<θ̄≤0.6–0.7, Ediff rising from 0.3 eV at θ̄=0 to 0.8 eV at θ̄=0.6. This is tentatively explained in terms of activated hopping into the almost empty second layer, followed by rapid diffusion, terminated by recombination of the excited atom with a hole in the first layer. At θ̄=0.8–1.0, Ediff falls sharply, corresponding to diffusion in the second layer. It is suggested that K adsorption sites are relatively weakly structure dependent, so that a displacement of atoms from the initially occupied, optimal sites occurs at θ>∼0.4 in order to accommodate more adsorbate in some direct contact with the substrate. The experimental results are shown to be consistent with a model based on delocalized bonding, i.e., metallic adsorption.