| dc.description |
The diffusivity of single atoms of Ta, Mo, W, Re, Ir, and Pt, adsorbed on clean tungsten surfaces, was studied by field ion microscopy. Activation energies for diffusion of third transition series metals on (110), (211), and (321) surfaces increase with atomic number to Re, then decline, confirming the importance of 5d-electrons in the adatom bonding. Except for Re, adatom diffusion on (211) surfaces at low temperatures is characterized by low activation energies and very low values of D<sub>0</sub> in the range 10<sup>−5</sup>-10<sup>−7</sup> cm<sup>2</sup> s<sup>−1</sup>. This is consistent with a facilitation of adatom displacement by correlated substrate motion. For all adsorbates, activation energies are larger for diffusion on (110) than on (211) surfaces, contrary to the predictions of pair-bond model calculations. This, and the restricted and directional bonding in Re, Ir, and Pt adatom clusters on (110) surfaces, are consistent with more localized bonding of adatoms on (110) than on (211) or (321) tungsten surfaces. |
