Seeing atoms: the origins of local contrast in field-ion images

Abstract

This article review the theory of local contrast in field-ion images, as it has developed over the last 15-20 years. The issue has been: how do we see atoms? The question has partly been whether contrast formation is dominated by a rate-constant or by a gas-concentration mechanism. But there has also been difficulty in establishing any satisfactory numerical explanation of the long-known resolving power of the field-ion microscope. Views on these issues have fluctuated, as understanding of the complicated physical situation has changed and as better models of various parts of the problem have been built. Recent work, involving new analyses of charged-surface models and JWKB-type rate-constant formulae, has much clarified the solution. It now seems probable that local contrast at normal operating temperatures (near 20K or above) is rate-constant determined, whereas local contrast at temperatures near 5K is dominated by imaging-gas distribution effects. Across-surface variations in the mean electric field in the forbidden ionization zone are very much greater than previously realised: the electron from the field-ionised atom has to tunnel through this zone, and these field variations lead to much greater rate-constant variations than previously estimated. Although much numerical modelling remains to be done, it is now possible to give a coherent, quantitatively justifiable, physical explanation of how the field-ion microscope works: we now know how men first saw atoms.