The Theoretical and Technical Development of Field-Ion Microscopy
A field-ion microscope is the most powerful microscopic device known today. It is the only instrument that can show directly the atomic structure of a specimen and the atomic lattice defects. But, for reasons that might lie in the difficulty of operation of the first instruments, perhaps the unorthodoxy of the principles involved, and a justified lack of commercial interest, it took a long time to be developed. When in the spring days of quantum mechanics Gamow1 (1928) explained the radioactive alpha decay as a tunneling effect, field-electron emission from metals was soon recognized by Fowler and Nordheim2 as another example of barrier penetration and simultaneously Oppenheimer3 suggested that the effect of field ionization of free atoms could occur when an electron would tunnel out in the presence of an electric field. While the first two effects commanded considerable interest, field ionization from the ground state of an atom was experimentally inaccessible because of the magnitude of the fields required. Handling large fields became a possibility with the introduction of the field-emission microscope in 1936.4 With the discovery of field desorption5 from a positive-point electrode the field range beyond 100 MV/cm, in which all effects of interest to us are taking place, was entered for the first time. The realization that the resolution limit of the field-electron microscope6 is determined by the tangential velocity of the emitted electrons and, to a lesser extent, by their de Broglie wavelength, which cannot be controlled under the prevailing conditions, led in 1951 to successful imaging of the emitter surface with positive ions rather than electrons.7 Atomic resolution was thus achieved for the first time.
KeywordsField Ionization Field Evaporation Proper Operating Condition Evacuate Glass Tube Lateral Velocity Component
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