Direct Measurement of the Pair Distribution Function for Adatoms on a Surface

The distribution function governing the equilibrium separation of two atoms on a single crystal plane has been measured for the first time. Quantitative determinations for tungsten atoms self-adsorbed on the (211) and (321) planes of tungsten reveal that, for interatomic distances $R>\mathrm{}7\mathrm{}$ Å, the behavior of the atoms conforms to that of independent particles. Long-range interactions are below the detection limit of $\approx \frac{\mathrm{kT}}{2}$, where $T$ is the equilibration temperature, in agreement with present theories of interatomic interactions.