On the problem of establishing the location of the electrical surface (image plane) for a tungsten field emitter

This paper discusses the problem of establishing the position of the electrical reference surface, relative to the positions of the atomic nuclei in a flat crystallographic surface, for the refractory metal tungsten that is widely used in field-ion emission experiments. Experimental results produced by the technique of Field Ion Appearance Energy Spectroscopy (FIAES) are of particular relevance. The paper first summarizes the state of existing theoretical models. It then re-examines how FIAES results can be used to derive estimates of the bonding distance for an adsorbed helium atom, and hence estimates of the electrical surface position. Modifications in existing data-interpretation procedures, suggested as a result of recent work on field adsorption, are explored. New empirical estimates of bonding distance and electrical surface position are thereby derived. A misfit between the systems for which advanced ab initio quantum-mechanical calculations exist and the systems used in the experimental work is noted, and a theoretical challenge is proposed.