BINDING AND FIELD DESORPTION OF INDIVIDUAL TUNGSTEN ATOMS

The binding energy of individual tungsten atoms on different planes of a tungsten crystal, heretofore inaccessible to measurement, has been explored by field desorption at 20°K. Tunneling, rather than evaporation over a Schottky saddle, is the limiting step under these conditions. The appropriate relations between desorption field and atomic binding are derived and tested by establishing the evaporation field for tungsten at 6.1 V/Å. Desorption measurements on single adatoms lead to the following binding energies: (110), 5.3 eV; (211), 7.0; (310), 6.7; (111), 6.0; (321), 6.7; (411), 6.2. On low index planes such as (110), (211), and (310), agreement with the energetics estimated from Morse and Lennard-Jones potentials is reasonable. For rougher surfaces, however, the experimental quantities are significantly smaller than expected, suggesting a fundamental limitation on such potentials. Comparison of binding energies with activation energies for diffusion over the (211) and (321) planes also leads to the view that fluctuations in the position of lattice atoms play an important role in surface migration.