The theory of diffusion and the principal methods of determining surface diffusion coefficients are presented and their strengths and weaknesses discussed. A summary of major experimental results for diffusion of metallic and non-metallic adsorbates on metal surfaces is given and areas of agreement and disagreement between various measurements are discussed. A brief overview of principal conclusions, problem areas and future directions concludes the review.

The effect of low-energy (15–200-eV) electrons on hydrogen, oxygen, carbon monoxide, and barium adsorbed on tungsten has been investigated by a field-emission technique. Desorption cross sections σ were determined from work function and Fowler—Nordheim pre-exponential changes and are significantly smaller than would be expected for comparable molecular processes. Marked variations in cross section with binding mode within a given system were found. Thus σH=3.5 10—20 cm2 and 5×10—21 cm2 for processes tentatively interpreted as the splitting of molecularly adsorbed H2 and desorption of H, respectively; σ0=4.5×10—19 cm2 for a loosely bound state and σ0≤2×10—21 cm2 for all other states; σBa<2×10—22 cm2 under all conditions. In the case of CO (reported in detail elsewhere), three binding modes observed previously could be confirmed and differentiated by their different cross sections: σvirgin=3×10—19 cm2; σβ=5.8×10—21 cm2, σα=3×10—18 cm2; conversion by electrons of virgin to β σvβ≥10—19 cm2. These results are interpreted in terms of transitions from the adsorbed ground state to repulsive portions of excited states, followed by de-exciting transitions which prevent desorption. Arguments are made to show that the excitation cross sections should be essentially ``normal,'' i.e., ∼10—16 to 10—17 cm2, and that the much smaller over-all cross sections observed are due to high transition probabilities to the ground state, estimated as 1014 to 1015 sec—1. A detailed calculation for the case of exponentially varying transition probabilities and repulsive upper states is presented and discussed, and the variations in cross section with binding mode made plausible. It is shown that low-energy electron impact constitutes a sensitive tool for studying chemisorption.

The concept of absolute half-cell emf is discussed and defined as VMS-φM for the reaction M→M+(solution)+e−(M), where VMS is the electrostatic potential difference between metal electrode and solution and φM the work function of metal in contact with solution. It is shown that this quantity is equal to VRS-φR, where VRS is the electrostatic potential difference between a reference electrode in air above the solution and the solution, and φR the work function in air of this reference. The quantity VRS′, the potential difference between reference electrode and the solution surface, was found experimentally by the vibrating condenser method for a number of half-cells, and φR was determined photoelectrically. It is shown from the variation of VRS′ with electrolyte concentration that the potential difference betwen the bulk of pure H2O and its air interface is ∼0.05 V, the surface being negative relative to bulk, and that this potential is increasingly screened out as electrolyte concentration increases. From these results for several different half-cells the absolute value of the standard half-cell emf for H2→2H+ is found to be −4.73±0.05 V. This result permits the calculation of single ion free energies of solvation. It is shown that the simple Born model as used by Latimer, Pitzer, and Slansky works remarkably well for simple cations, including polyvalent ones, and for spherical anions, but breaks down for complex anions like OH−, NO−3, etc. Ions in which chemical bonding effects to the solvent play an important role show anomalously high solvation energies. The solvation energy of H+ is −10.98 eV in H2O and varies very little from this value in several different solvents, suggesting that free H+ may predominate in these solvents. This could result from the fact that the small effective radius of free H+ leads to a greater solvation energy than the combination of bond formation and solvation of the resultant much larger ion (H solvent)+. Similar arguments can be used to explain why for instance Fe3+ is not reduced by H2O despite the fact that the third ionization potential of Fe is 30 and that of water 12.6 eV. Other possible applications of the method used in these experiments are discussed.

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Field emission consists of electron tunneling from a conductor under the influence of a high applied electric field, which deforms the potential barrier at its surface (Figure 1). Usually the medium into which tunneling occurs is a vacuum; it is quite feasible, however, t o obtain tunneling into normally insulating liquids or through liquid-like films adsorbed on the emitter. Such experiments can yield information on the heat of solution of electrons in condensed phases. Field ionization consists of field emission from atoms or molecules, Le., tunneling of electrons from these species under the influence of high fields. Again the process, normally carried out in a lowpressure gas, can occur in liquids and films. It can then yield information on the energy of filled levels, ie., the valence bands of such media.Field emission and field ionization in liquids also provide controlled high-intensity sources of electrons and positive ions in liquids and readily yield the mobilities of the charge carriers. I n addition, both reveal a number of high-field phenomena difficult or impossible to see by other means. By providing accurately known high fields and extremely well-characterized electrodes, they also shed light on breakdown phenomena in liquids. With the exception of field emission through Ne, Ar, Kr, and Xc layers' the subject matter of this paper is based mainly on recent work of Halpern and the author.*v3We start with a very brief rcvicw of basic t h e~r y .~ For present purposes it suffices to consider a metal as a potential well ; because of the exclusion principle, only two electrons can occupy each translational level, so that the well will be filled to a depth of several electron volts, as shown in Figure 1. The highest filled level (Fermi level) is still scveral electron volts below the vacuum; this energy barrier is called the work function, 4. I n thermionic emission electrons must be excited over this barrier; in field emission the application of a strong electric field deforms it, as shown in Figure lb, so that elcctrons can tunnel through the resultant POtcntial hill. The tunneling probability D is given with Robert Gomer was born in Vienna, Austria. H e received his B.A. from Pomona College in 1944, spent two years in the A r m y , and received his Ph.D. from The University of Rochester in 1949. After a year's postdoctoral work, he joined the staff at The University of Chicago, where he has been teaching since. H i s research interests center o n the chemistry and physics of surfaces. The subject of this Account i s a departure f r o m his m a i n field of interest, but is related to it by more than experimental method, as the section on films indicates. reasonable accuracy by the WKB approximation Dexp( -2L'k'dz) where k' = ( 2 m 8 / f i y y v -B)l'Z (2) and the integral extends from one end of the barrier to the other.I n a metal most of the emission comes from the vicinity of the Fermi level for which the barrier height is the work function 4 ; it is not hard to show that the dominant term in the F...