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 $\text{θ≳0.6}$ 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 ${\text{θ̄≈0\hspace{0.17em}n}}_{110}{\text{/n̄=40,n}}_{211}{\text{/n̄=0.35,n}}_{111}{\text{/n̄<0.04,n}}_{100}\text{/n̄<0.01}$ . As θ increases the anisotropies decrease, and for $\text{θ̄≳0.5}$ the ratios $\text{n̄→1}$ for all regions except 100. The corresponding differences in heats of adsorption at $\text{θ̄≈0}$ 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.