Field Adsorption and Charged Surfaces

Abstract

This thesis deals with the theory of charged surfaces, field adsorption and field-ion imaging. The existing literature dealing with these subjects is in many ways inconsistent and confusing, and contains conceptual errors, After a brief review of existing ideas, we put forward self-consistent definitions for various parameters, and formulate new models for charged metal surfaces and for field adsorption, The model can be treated analytically, and incorporates the requirements that the crystallographic surface structure, and the localised charge distribution at the emitter surface, be taken into consideration. This model is applied to a paradigm system, namely helium on tungsten (111). Field and potential variations above a charged surface are explored and it is shown that at a temperature of 80K the field variation in the critical surface is sufficient to explain field-ion image contrast only if a field-adsorbed layer is present. It is also shown that the equipotentials above the planar emitter surface can be imagined to have an egg-box shape, and that the higher the value of the proper polarizability of the surface atoms the further from the emitter surface will be an equipotential of given potential value. An interesting consequence of this is that higher surface-atom polarizability can lead to reduced image contrast. In the studies of field-adsorption binding energy, the insufficiency of previous treatmentSis demonstrated. In particular it is shown that neglecting depolarization effects due to mutual interaction between surface atoms leads to marked over-estimation of the binding energy; mutual depolarization in fact means that in some close-packed planes it is impossible to have a fully adsorbed layer. It is also shown that for He on W(111) we cannot have a complete second field-adsorbed layer on top of the first. When all corrections are taken into account, we have estimated that for the best image field for Helium (45 V/nm), the short-range binding energy for He on W(111) probably lies between 25 and 50 meV. It is concluded that this low value is not sufficient to explain field adsorption and that further modification to our model toward a more realistic model is needed in the future.