Adsorbate overlayer

This finally leads to the (T, a, A< ) phase diagram shown in Fig. 5.10 (Plate 5.1). The plot in Fig. 5.10a shows the modified interfacial free energy y" of different adsorbate overlayers as a function of the water chemical potential and the electrode... 

This combination of techniques allows us to determine the structure of the adsorbed species while on the metal surface and after desorption into the gas phase. Furthermore, molecular rearrangements in the adsorbed overlayer as a function of both the substrate temperature and background pressure can be studied. 

The technique of low energy electron diffraction (LEED) has been the most widely used tool in the study of surface structure. LEED experiments involve the scattering of monoenergetic and collimated electrons from a crystal surface and detection of elastically diffracted electrons in a backscattering geometry (Figure 2). The characteristic diffraction pattern in LEED arises from constructive interference of electrons when scattered from ordered atomic positions. The diffraction pattern represents a reciprocal map of surface periodicities and allows access to surface unit cell size and orientation. Changes in the diffraction pattern from that of a clean surface can be indicative of surface reconstruction or adsorbed overlayers. 

Imaging the landscapes of solid surfaces ex-situ and in-situ is nowadays an established method for gaining information about their local structure. Reliable data about the structure of adsorbate overlayers on top of the substrate can also be obtained by this technique (see other papers in this volume), if the atoms are strongly adsorbed. What to... 

In chemically reactive adsorbed overlayers, the probabilities of arrangements of adsorbed particles and accordingly the reaction rate are defined by the interplay between adsorption, reaction, and adsorbate diffusion. The activation energy for surface diffusion is often relatively low, therefore the adsorption overlayer is close to equilibrium giving a framework for analysis as presented above. When diffusion of some of the reactants is slow compared to other steps the arrangements of adsorbed particles is often far from equilibrium. In particular, immobile reactants may form islands (Figure 3.18). 

Competitive adsorption and mobility of adsorbates on the surface attribute to the coadsorption-induced reconstmction of adsorbate overlayers. If surface species is immobile because of a high activation energy for surface diffusion, coadsorption cannot take place. On the other hand, the adsorption energy of one adsorbate must be sufficient to compress the other adsorbate into a more compact layer. If coadsorbates have a very low heat of adsorption, the thermodynamic driving force for adsorbate overlayer reconstruction is absent. 

At low coverages, most metallic adsorbates form ordered overlayers with a (1 x 1) surface structure on metal substrates. This implies that the substrate acts as a template and has a significant influence on the growth mode of the deposited material. This effect is usually called epitaxial growth. A more restricted definition of epitaxial growth would include only those examples where the substrate imposes its own crystal structure, orientation, and lattice parameter on the adsorbed overlayer. (This restricted definition is also called pseudomorphic growth.)... 

The system 0/Gd/W(l 10) exhibits a significantly different behavior for angles smaller than 20°. A pronounced feature overlaps the general shape of the asymmetry curve an additional change of sign and a high maximum of 15% at = —10° (deep minimum of —18% at 10°, respectively) occurs. This observation directly points to different chemical or electronic properties in the adsorbate overlayer. 

With increasing film thickness the electronic structure of the adsorbate overlayer becomes undistinguishable to that of the bulk material this will be demonstrated for Fe evaporated on W(llO) [13]. In the following it will be shown that a thickness of 20 layers is already sufficient to map the transitions in the spin resolved bulk band structure of Fe(llO). The analogous behavior was found for Co(OOOl) films on W(llO) [14]. 

Fig. 5.24 Layer projected density of states for oxygen on Fe(llO) top) and clean Fe(llO) bottom) (reprinted with permission from [74]. Copyright 1993, American Institute of Physics). The solid lines represent majority and dashed lines minority electrons. Fe(S) denoted the topmost Fe layer, Fe(C) the iron center layer, and O the oxygen adsorbate overlayer...

Vacuum compatible solids. PIES is particularly useful for studying molecular adsorbate overlayer geometry on metallic single crystals. 

Characterization of adsorbate overlayers measuring techniques (Ch woll)... 

Characterization of adsorbate overlayers Measuring techniques P. Zeppenfeld... 

The final section in Chapter 2 deals with the molecular asp>ects of transition-metal catalysis. It serves as an introduction to Chapter 3. A characteristic feature of the transition-metal surfaces under catalytic conditions is their potential to restructure. Adsorbate overlayer adsorption can induce the surface to reconstruct with rapid diffusion of the metal as well as the overlayer atoms. The state of the surface may start to resemble that of a solid state compound. The state of the surface is not only strongly influenced by the composition of the reactant gas, but can also be strongly affected by the addition of promoters or other modifiers, that can result in alloy formation or new complex surface phases. 

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