On the atomic resolution of a field ion microscope

The effects of tip temperature (T_T), imaging gas mixture, and electric field strength on the resolution of the field ion microscope have been measured quantitatively. The image diameter of individual tungsten atoms had a quadratic dependence on T_T between 11 °K and 62°K. This result was in agreement with the theory of resolution which predicted that the transverse component of the velocity of the imaging ion limited the resolution of the FIM. The data also indicated that the imaging gas atom was not accommodated fully to T_T when it was ionized. The degree of thermal accommodation of the imaging gas atom prior to its ionization was increased as a result of imaging in a 10 % neon-helium gas mixture as opposed to imaging with pure helium gas. The extrapolated value of the experimental contribution to the image size at 0°K was larger than the value predicted by the existing theory. This discrepancy was attributed to the neglect, in the existing model of resolution, of the nature of the local electric field strength in the region of space where the imaging atom was ionized. The dependence of image size of an atom at constant T_T exhibited a maximum. This result was at variance with the existing theory of atomic resolution. An explanation of this effect in terms of the field dependence of the equilibrium concentration of gas atoms at the surface of the tip was given.