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The preceding papers of this symposium have shown how much of our knowledge on surfaces has been derived from the interpretation of molecular processes at the gas-solid interface. Most of these experiments are of a rather macroscopic nature. With the field emission microscope and particularly with its more recent version, the field ion microscope, we now have research tools that permit direct visual observations of molecular adsorption films and of the atomic structure of metal surfaces. In this paper the field emission microscope will be discussed only briefly to outline its range of application and to show the progress made possible by the introduction of an imaging process with positive ions. The results of field ion microscopy of atomic surface structures of metals will then be represented in more detail.
The field emission microscope (1) displays on a fluorescent screen the surface of a specimen shaped as a fine needle tip. The image is formed by radial projection of field-emitted electrons, which results in a magnification equal to the ratio of screen distance to tip radius. Magnifications of 105 to 106 are typical. The fine tips can be prepared by chemical etching of wires, and the finishing of the tip to an almost perfect hemisphere is done by annealing the tip.
The crystallographic emission patterns of a number of high-melting metals have been studied, either in the clean state or with adsorption films of various gases (2). Established techniques are: rate measurements of surface migration (3), the utilization of the wide temperature range by immersing the entire microscope into a cryogenic bath (2), the evaluation of work functions of specific crystal planes by using Fowler-Nordheim plots (2), and the measurements of binding energies of physisorbed or chemisorbed gases by gradual thermal or field desorption (4). Progress in recent years was made by applying pulsed fields (5) for the elimination of the field effect on surface migration, the use of an a-c field desorption technique (6), the in situ growth of whisker tips (7) to study non-refractory metals, and more quantitative studies of gas adsorption, mostly on tungsten. A number of comprehensive review articles are available (2,8). Merely as an illustration some field emission patterns are shown in Figs. 1 to 3.
Müller, E. W.
Research Professor of Physics, The Pennsylvania State University, University Park, Pa.