Adatom-surface atom exchange

K). The changes in slope indicate changes in the mechanism of surface diffusion. While, at low temperatures, adatom diffusion or adatom-surface atom exchange appears to be the dominant atom-transport mechanism, the diffusion of surface vacancies created by thermal roughening is likely to be dominant at high temperatures to account for the increased activation energies. For example, copper adatom and vacancy diffusion rates in the Cu(l 10) crystal face are given by... 

The simple hopping mechanism, however, is not the only possible way for atoms to move on Cu(100). Figure 6.11 illustrates another possible diffusion process known as an exchange mechanism. In this mechanism, the adatom (shown in dark grey), replaces an atom that is initially a surface atom (shown in light grey) while the surface atom pops up onto the surface,... 

The interpretation of this data on metals in terms of microscopic mechanisms of surface atom transport is not totally understood. The original papers[ 11] proposed that during surface transport the controlling process was adatom terrace diffusion between steps with the adatom concentration being that in local equilibrium with the atomic steps. This may indeed be the case, but in light of other experiments on adatom diffusion[13] and exchange processes at steps[14] the possibility of step attachment/detachment limited kinetics caimot be raled out. 

In summary, Type I surface alloys on more densely packed surfaces can be formed at temperatures where horizontal adatom difiusion and vertical exchange between adatoms and surface atoms are sufficiently fast. Vacancy diffusion in the alloyed layer may contribute to the intermixing, but once vertical exchange processes are activated, adatom diffusion will be sufficiently fast anyway and will... 

Hence, for both the Ru/Pt(lll) and Pt/Ru(0001) systems, there is a temperature window for effective exchange between adatoms and surface atoms and adatom diffusion (Figure 12.9) and both systems form an alloyed thin film. Owing to the lower surface energy of Pt as compared to Ru, however, this film is exposed on the Ru(OOOl) surface but capped by a Pt-rich layer on Pt(lll) [38]. 

Figure 3 A cross-sectional view of an exchange process of indium (bright) with a copper (dark) adatom, leading to a long jump of the indium atom, (a) A copper adatom arrives at die embedded indium site, either through normal hopping or exchange hopping (not shown), (b) The copper adatom changes places with die indium atom, (c) The indium adatom now makes one or more hops over the surface before it reinserts itself into the first layer. A multiple encounter between the copper adatom and the indium may lead to even larger displacements.

In what follows, each configuration will be identified by a simple notation describing the most salient features that defines it. All atoms will be identified by their chemical identity and the plane in which they are located overlayer (O), surface (S), first plane below the surface (lb), etc. Exchanges between adatoms... 

H2/D2 exchange involves type (/) species, but their exact role is still under debate. A kinetic isotope effect operates such that HD is produced — 1.5 times faster from a 2H2 D2 mixture than from a H2 2D2 mixture. Naito et al favour an Eley-Rideal mechanism at 200 K involving a gas-phase molecule and a type (/) chemisorbed atom. Kokes et a/. initially favoured a similar mechanism involving a type Hi) chemisorbed molecule instead of one in the gas phase. However, in a later paper they show that type Hi) species are only important in allotropic ortho-para) conversion and that the exchange reaction involves a Bonhoeffer-Farkas mechanism, using type (/) adatoms, at all temperatures. Richard et al., on the basis of the variation of the first-order rate constant with pressure, deduced that the mechanism must involve atomic H on surface pair sites that most probably were adjacent ion pairs thus, a Rideal type of mechanism was favoured. 

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