Field Emission through Atoms Adsorbed on a Metal Surface

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Abstract
An exactly solvable one-dimensional pseudopotential model is employed to calculate the field-emission probability and current from a free-electron metal through both metallic and neutral absorbates. The adsorbate potential is taken to be atomistic in nature consisting of an attractive square well plus a delta-function (orthogonalization) core outside the surface of the metal. Metallic adsorbates lead to the wide (Δ∼1-eV) resonances in the emission probability, an additional peak or shoulder in the energy distributions, large (102−104) enhancements in the emission current, and reductions in the slope of the Fowler—Nordheim plots of In[current/(field)2] versus (field)−1 at fields F≳5×107 eV/cm. Neutral adsorbate potentials without bound states lead to reductions in both the emission probability and current, and to a simple scaling of the Fowler—Nordheim energy distributions. Neutral adsorbate potentials with bound states below the metallic conduction band lead to enhancements of the current for loose binding, additional asymmetry in the energy distributions for all strengths of binding, and to both reductions in the current and strong field reductions of the slope of the Fowler—Nordheim plots for tight binding. The reduction in the emission current by neutral adsorbates is used to interpret the simultaneous reduction of the work function and emission current by the adsorption of nitrogen on the (100) and (411) faces of tungsten. The prediction for metallic adsorbates of an extra peak in the energy distributions whose location shifts linearly in F distinguishes the results of the atomistic pseudopotential model from those of a surface-film model.

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