Adsorption of Potassium on Tungsten: Measurements on Single-Crystal Planes

Work functions of the (110), (211), (100), and (111) regions of a thermally annealed tungsten field emitter were measured as a function of potassium coverage. Immobilely adsorbed layers yielded absolute dipole moments, permitting estimates of adsorbate charge. The results at low coverage are μ110=15.7 D, μ211=13.6 D, μ100=11.5 D, μ111=9.5 D, with μ defined as 2qd0=Δφ/2πn. At coverages θ≳0.6 there is evidence of higher layer formation and field enhancement. Work functions after thermal equilibration in combination with φ-vs-n data for immobilely adsorbed deposits yielded coverage anisotropies under equilibrium conditions. At θ̄≈0 n110/n̄=40,n211/n̄=0.35,n111/n̄<0.04,n100/n̄<0.01. As θ increases the anisotropies decrease, and for θ̄≳0.5 the ratios n̄→1 for all regions except 100. The corresponding differences in heats of adsorption at θ̄≈0 are less than 0.4 eV for all regions except possibly 100, with binding strongest on 110. A quantum-mechanical model is briefly sketched, and a quasistatistical treatment of alkali adsorption is outlined. It is shown that the small differences in H for substrate regions of considerably different work function and structure can result from the near cancellation of variations in explicitly electrostatic effects and exchange effects. The φ-vs-n curves on single planes can be fitted by a simple depolarization model leading to values of the ad-complex polarizability of 26 Å on (110), 46 Å on (211), 35 Å on (100), and 29 Å on (111). It is shown that the concept of ad-complex polarizability follows naturally from the statistical model.