Increased Image Brightness Caused by Field Adsorption in Field-Ion Microscopy

We explain the observations that field-adsorbed rare-gas atoms increase the ionization rate near the tip in the field-ion microscope. This increase results from exchange effects associated with antisymmetrization of the electronic states appearing in the time-dependent perturbation theory for the ionization process. Other effects, such as resonance associated with the adsorbate potential well, are unimportant. We obtain order-of-magnitude agreement with experiment, although the predicted field dependence of the enhancement does not agree with Rendulic's observations.