IVitli field emission electron microscopes one can see the structure of the surface of a single crystal a t the tip of a metal "point." The niagnification is about lo6 and the resolution about 20 X 10-8 cm. At 2800' K., the surface of W point is heniispherioal. Only the 110, 111 and 100 regions consist of small flat planes. In fields of 50 million volts/cm. and at 1200'K. these planes enlarge. The edges of the planes are seen to be in violent agihtion. Hence surface atoms are mobile a t temperatures above one-third of the melting point. Ba atoms show surface mobility a t 400°K. on the 110 and at 800°K. on the 100 planes. With the flash filament technique one can measure the rate a t which Nz is adsorbed on a W ribbon a t low pressures. After NS has been adsorbed for minutes a t a low temperature, the ribbon is flashed a t 2300°K. The sudden rise of pressure is recorded with an ion gage and measures 0, the layers adsorbed. From a family of e us. time curves one calculates s, the sticking probability. For T = 300'K., s = 0.55 from e = 0 to 1.0. Then s decreases from 0.55 to about 4 X for e = 2.0. These data yield an activation energy for the conversion of a molecule to two adatoms of about 100 cal./g.mole for e = 0 to 1.0 and 5000 cal./g. mole at 0 = 2.0. Other experiments yield 100,000 cal./g. mole for the heat of adsorption of 2 adatoms. The heat of adsorption of molecules is much smaller.In recent times two new tools have been developed which should be of considerable interest to the physical chemist in the study of adsorption on metals. The first is the field emission electron microscope in which the electron emission from a small hemispherical surface of a single crystal is projected onto a fluorescent screen where it portrays an image of the surface magnified about a millionfold. With it one can see that certain crystallographic planes tend to grow more readily than others, that the surface atoms are mobile at temperatures of about one-third the absolute melting point, and that the ease of mobility depends on the crystal plane. When controlled amounts of a second material are adsorbed on this hemispherical surface one can ascertain whether they are preferentially adsorbed on certain planes, the directions in which they migrate on different planes, and the temperatures a t which migration and evaporation occur a t observable rates. For relatively small flat molecules such as phthalocyanine, the apparent positions of the benzene rings have been observed. For unusually fine '(points," which yield magnifications of ten million, it may be possible to see the splitting up of an adsorbed molecule into atoms and how the rate of this process depends on temperature.The second technique employs the recently improved ionization gage which measures pressures from 10-lO to mm. and permits the recording of pressure changes in about 0.1 second. Because of the sensitivity and the speed of this gage it is possible to measure rates of adsorption from metal surfaces with an area of only one square cm. and at pressures so low tha...

Low-energy electron diffraction, with the diffracted electrons post-accelerated and observed on a fluorescent screen, has been used to study the adsorption of oxygen on a (100) face of a nickel crystal. Upon admitting oxygen to a clean face at room temperature, or at elevated temperatures up to 350°C, the first adsorbed atoms are arranged in narrow bands parallel to [l1OJ directions on the crystal surface. Within one of these bands, atoms lie on lines at right angles to the band; these lines have a uniform separation of 4.98 A, but the atoms along each line are somewhat randomly spaced in multiples of the nickel spacing of 2.49 A. With further oxygen exposure (30X 10-6 mm Hg sec) sufficient to produce half of a monolayer (one oxygen atom for every four surface nickel atoms), the arrangement is very chaotic unless the crystal has been heated, but after heating the arrangement is a simple 4 R. E. Schlier and H. E. Farnsworth, Advances in Catalysis 9, 434 (1957).

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Nickel (110) surfaces, in clean and nearly clean conditions, have been studied by low-energy electron diffraction. When free from foreign atoms, the surface (110) plane of atoms has the normal arrangement expected for such a plane. Very slight contamination by oxygen (and perhaps other atoms) results, in some cases, in an arrangement of the topmost layer of atoms in which nickel and oxygen (or another atom) alternate along each [100] line and also along each [110] line. Study of such a surface has shown that the superficial half-layer of nickel atoms is displaced toward the bulk of the crystal by about 0.10 A.

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