Surface self-diffusion by ion impact

Abstract

A recent study of electrical discharges in vacuum has led to the hypothesis that a surface self-diffusion by ion impact should exist and play an important role in the discharge initiation. In order to check the existence of this diffusion, we have (1) interpreted former experiments ( Ar + W , Ta + Ta ) and (2) studied the effect of hydrogen ion impact on a tungsten single crystal in a field electron microscope. The ions are produced by collisions of the field electrons with gas molecules in the pressure range from 10 −6to 10 −2 Ton. The experiments show that the ion impact produce a tungsten surface atom transport in a field gradient which leads to a formation of protrusions. This growth — which occurs reproducibly — indicates the existence of this diffusion. The time of the protrusion growth to a given size is determined by measuring the increase of the field electron current. The number of atoms transported is proportional to the number of impinging ion per cm 2. A determination of the surface diffusion coefficient by ion impact is attempted by comparing the ion impact diffusion with thermal surface diffusion.