Study on the local barrier field variations for electron tunneling in field ionization at a step edge of W(112) with a micro-probe hole field ion microscope

The local field ion densities of W(112) surface have been observed using a micro-probe hole field ion microscope combined with a pulse counting analysis. The mapped field ion densities clarified the anisotropic emission property within a single bright spot of a surface W atom site at a step edge. Based on the most relevant formula for tunneling barrier strength provided by B. Halpern and R. Gomer, we derived the local "across-atom" barrier field variations in the same step edge area. We found that the location of maximum barrier field strength is noticeably shifted towards the step side, rather than being on the top of the substrate surface atom site. The result indicates that the locus in which most highly promoted field ionization events take place is much smaller than previously estimated because of the extremely localized barrier field produced by the field adsorption of helium imaging gas atom.