FIELD EiVIISSIOX MICROSCOPY 



num and vanadium tips, while the same trons to depict the emitter tip. The micro- 

 molecules show up as doublets on tantalum, scope tube is operated with the tip at a 

 rhenium or platinum emitters. positive potential, and it is filled with a few 



Very little work has been done so far with microns of gas, preferably helium. At a field 



adsorption on metals other than tungsten, of about 450 MV/cm, e.g., more than 10 



Exploratory investigations were made with times higher than needed for field electron 



tips of Re, Ta, Mo, Nb, Ir, Pt, Zr, Ti, Ni, emission from a negative tip, heUum atoms 



Fe, V, Cu and Ag. Some chemical reactions coming near the tip surface will be ionized 



were studied such as the reduction of oxygen by tunneling out an electron. The helium 



films by hydrogen, the oxidation of carbon ions thus formed travel to the screen on the 



films, the formation of tungsten and molyb- same radial trajectories as the electrons in 



denum carbides and silicides and the am- the field electron microscope, producing 



monia synthesis on platinum. Recently the there an ion image of the tip. With the field 



soft metals became available as emitter tips properly adjusted ionization will only occur 



by using whiskers groAvn in situ; however, so where the local field is slightly enhanced due 



far only patterns of clean surfaces of Fe, Cu, to the protrusion of the metal atoms of the 



Au, Ag, Al, Ge and Hg have been studied by surface. Because of the attraction of the 



this method. helium atoms that are polarized in the highly 



Another promising new techniciue for inhomogeneous field, the approaching atoms 



either the preparation of perfectly clean travel so fast that only a few are ionized, 



surfaces without the application of heat or Most of them touch the tip surface, accom- 



for obtaining new data on the binding energy modate to its low temperature and rebound 



of adsorbates is desorption by a reversed only very slowly by performing a hopping 



field. Field desorption (1) of a barium fihn motion up to a few angstroms above the 



from tungsten requires about 100 million surface, until they become ionized in the 



volts per centimeter at room temperature, locally enhanced field above a protrusion, 



the exact value depending upon the degree Then they take off to the screen. The great 



of coverage and the crystal plane, while the improvement in resolution, compared to the 



removal of an oxj^gen film needs as much as field electron microscope, is possible because 



400 Mv/cm. of the small random tangential velocity com- 



A basic difficulty of field electron micros- ponent (equivalent to kTvip) and because of 



copy is that only the ratio of (i>^i'^/F can be the short de Broglie wavelength associated 



derived from measurements of current den- wdth the relatively heavy ion. For the first 



sity, which makes the interpretation not un- time in the history of microscopy the indi- 



ambiguous. Rate determinations for finding vidual atoms as they constitute the lattice 



activation energies may be influenced by the of metals have become visible. The theoret- 



electric field present during the obsen''ation, ical resolution is about 1.5 A, and experi- 



although the field effect is usually .small and mentally the individual atoms on the 111 



may sometimes be eliminated by observing plane of silicon, with a triangular spacing of 



with microsecond pulses (4) rather than 2.8 A have been resolved visually. Adjacent 



with d.c. tungsten or platinum atoms, 2.74 and 2.77 A 



apart, have been photographed. The image 



1 he I' leld Ion Microscope intensity is low, corresponding to an ion 



A most promising new research tool is the current of 10~^ amp on a 4-inch image screen, 



field ion microscope (1, 5, 6), which is a so that a typical exposure time is one min- 



modification of the field emission microscope ute. 



in that it uses positive ions rather than elec- Field ion microscopy is apphcable onl}- to 



329 



