STUDY OF CO SURFACE-DIFFUSION ON CO W(111) BY ANALYSIS OF CO+ FIELD-ION RATE FLUCTUATIONS/

An analysis of field ion rate fluctuations is proposed for studying th e mechanism of the reactive imaging gas supply in a field ion microsco pe (FIM), In the experiment CO+ ions emitted from a few selected surfa ce sites of the apex plane of a [111]-oriented W tip and mass-to-charg e resolved in a magnetic sector field are registered. The CO+ ion rate fluctuations are analyzed by a method developed by Gomer for analyzin g field electron current fluctuations. The CO gas supply toward the pr obed surface sites is found to have a diffusion character, confirming the role of the weakly bound CO layer in the localized field ionizatio n of CO used as imaging gas in a FIM.