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Model Studies of Collective Diffusion
Non-Equilibrium Effects in Diffusion Barriers
Abstract Since the pioneering surface diffusion experiments about 80 years ago [1], the field of surface diffusion has matured such that we have nowadays a fairly good understanding of how the adparticles migrate along the surface, and what principles govern diffusion processes [2–5]. This is possibly best illustrated by the low-temperature limit where adatoms often move by a sequence of jumps from one adsorption site to another. In these cases one often assumes that each jump is thermally activated and characterized by a well-defined activation barrier. Consequently, one also assumes that the diffusion coefficient can be described by an Arrhenius form, thus yielding some activation barrier for the diffusion process. Despite its common use, however, this approach is problematic for a number of reasons. Recent studies have demonstrated, for example, that interactions with other adatoms affect the potential energy surface felt by the tagged adatom [6, 7]. Therefore, even in the low-coverage limit [7], the diffusion barrier extracted from an Arrhenius analysis is some complex average of various activation barriers rather than a well-defined activation energy of some individual process. Another concern is the fact that there are many experimental techniques whose results for similar surface systems are not consistent. Instead, the results may strongly depend on the technique used, a fact whose reasons are not thoroughly understood. Obviously the differences may be due to variations in sample preparation or surface contamination, or due to the fact that some techniques such as scanning tunneling microscopy may interfere with the true diffusion process [8, 9]. On the other hand, the differences can also be explained by the fact that some methods probe the diffusion process strictly in equilibrium, while other techniques operate under far-from-equilibrium conditions. Indeed, various experimental studies have proposed [2, 10–13] that non-equilibrium measurements may yield results which are clearly different from the equilibrium ones.
Model Studies of Collective Diffusion
Non-Equilibrium Effects in Diffusion Barriers
Abstract Since the pioneering surface diffusion experiments about 80 years ago [1], the field of surface diffusion has matured such that we have nowadays a fairly good understanding of how the adparticles migrate along the surface, and what principles govern diffusion processes [2–5]. This is possibly best illustrated by the low-temperature limit where adatoms often move by a sequence of jumps from one adsorption site to another. In these cases one often assumes that each jump is thermally activated and characterized by a well-defined activation barrier. Consequently, one also assumes that the diffusion coefficient can be described by an Arrhenius form, thus yielding some activation barrier for the diffusion process. Despite its common use, however, this approach is problematic for a number of reasons. Recent studies have demonstrated, for example, that interactions with other adatoms affect the potential energy surface felt by the tagged adatom [6, 7]. Therefore, even in the low-coverage limit [7], the diffusion barrier extracted from an Arrhenius analysis is some complex average of various activation barriers rather than a well-defined activation energy of some individual process. Another concern is the fact that there are many experimental techniques whose results for similar surface systems are not consistent. Instead, the results may strongly depend on the technique used, a fact whose reasons are not thoroughly understood. Obviously the differences may be due to variations in sample preparation or surface contamination, or due to the fact that some techniques such as scanning tunneling microscopy may interfere with the true diffusion process [8, 9]. On the other hand, the differences can also be explained by the fact that some methods probe the diffusion process strictly in equilibrium, while other techniques operate under far-from-equilibrium conditions. Indeed, various experimental studies have proposed [2, 10–13] that non-equilibrium measurements may yield results which are clearly different from the equilibrium ones.
Model Studies of Collective Diffusion
Non-Equilibrium Effects in Diffusion Barriers
Vattulainen, I. (Autor:in)
01.01.2001
10 pages
Aufsatz/Kapitel (Buch)
Elektronische Ressource
Englisch
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