Adsorption of Barium on Tungsten: Measurements on Individual Crystal Planes

Abstract

The adsorption of barium on the (110), (211), (100), (111), and (221) planes of a tungsten field emitter was studied for both thermally equilibrated and unequilibrated adsorbate layers. At low coverage the equilibrated atom densities on the (110) and (221) planes are ∼100 times greater than on the (211) and (100) planes, while for $\text{θ̄>0.2}$ equilibrated atom densities are essentially identical on all planes. Dipole moments are roughly the same on all planes, corresponding to q∼1.0 electron/atom. At high coverages (θ∼0.8) there is evidence for rearrangement on heating to produce pronounced work function minima on the close‐packed crystal planes. These results are interpreted in terms of the previously suggested model of electropositive adsorption, with generally good agreement obtained. The influence of the small size of the substrate on the measurements is considered, and it is shown that these results should be identical to those on perfect macroscopic surfaces in most situations.