Ion impact on field emitter crystals

Overview & Synopsis

The hypothesis is introduced that the ions impact distribution on a crystal surface is anisotropic, in the case where the crystal is a field emitter tip, and the ions are produced by field electrons collisions with the residual gas. This anisotropy is a result of the fact that nearly all the ions which bombard the tip hemisphere are produced very near the surface, where the electron current is highly anisotropic. The energetical and geometrical distributions of the bombarding ions (as well as absolute ion currents) are determined theoretically on the basis of electron and ion trajectories calculated by Vernickel and Welter. The geometrical ion distribution at the surface should form an image similar to the field electron microscope image. This image should be visualisable by an observation of the bombardment defects in a field emission microscope. Preliminary experiments have been made with a tungsten tip bombarded by hydrogen ions. These experiments as well as an evaluation of known field emission micrographs seems to confirm the anisotropy hypothesis and indicate the existence of the ion impact image.