From a Tool to an Icon: The Ball Model and the Field Ion Micrograph

By Mari Yamaguchi

University of Tokyo


Abstract:

The Field Ion Microscope (FIM) is a type of electron microscope in which a high negative voltage is applied to a metal tip encased in an evacuated glass tube and positioned some distance from a glass screen coated with a fluorescent material. When a rarefied gas is introduced into the tube, the gas atoms, ionized by the tip, strike the screen and form a pattern of the tip surface. Each spot on the micrograph screen thus represents the image of an atom. In 1955, Erwin W. Mueller (1911–1977) successfully captured the FIM image of individual atoms. Mueller then developed a model with cork balls for indexing the FIM micrograph by comparing it to the expected structure of the tip surface derived from X-ray analysis. Then he painted the protruded sites of the model with a fluorescent dye and photographed it. After comparing the micrograph to the photograph, Mueller found out that the ball model was useful to interpret FIM images. Not many researchers followed Mueller’s constructional approach, since fabricating such three dimension models required too much time. After computer simulations became available, Mueller himself recognized the advantages of this method for producing lattice models, enabling more quantitative interpretation in much shorter time. Nevertheless, the ball model appeared along with Mueller in magazine and newspaper articles, and gradually gained recognition as an atomic structural icon.