Pattern, in LEED

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. 

In most - but by no means all - studies of binary alloy systems reported so far, qualitative LEED data indicate that the surface unit mesh corresponds to what expected from truncation of the bulk lattice [5]. The observation of the expected pattern in LEED in itself is no proof that the surface atomic structure is actually the bulk truncation one. Furthermore, in the case of ordered intermetallic compounds, the bulk termination model is not normally univocal since the plcuies stacked along a specific crystallographic direction do not necessarily have all the same composition. In the case of fee CusAu (LI2) ordered compounds (Fig. 1) all the crystallographic directions, except the (111) have an. ..ABAB... stacking with - for instance in the case of PtsSn - a plane of pure Pt alternating to a plane of composition PtSn. Both terminations correspond to bulk truncation and in both cases the composition of the outermost plane is different from the average one of the bulk. 

The "rosette" LEED patterns were successfully observed on Ag(lll) at potentials between the pea 2-2 and 3-3. The observation of the "rosette" patterns in LEED suggests that the iodine adlayer is more strongly attached on Ag( 111) than on Au( 111). It is now clear that the peaks 2-2 in Figure 4a correspond to the transition between the... 

In LEED experunents, the matrix M is detennined by visual inspection of the diffraction pattern, thereby defining the periodicity of the surface structure the relationship between surface lattice and diffraction pattern will be described in more detail in the next section. 

The diffraction pattern observed in LEED is one of the most connnonly used fingerprints of a surface structure. Witii XRD or other non-electron diffraction methods, there is no convenient detector tliat images in real time the corresponding diffraction pattern. Point-source methods, like PD, do not produce a convenient spot pattern, but a diffrise diffraction pattern that does not simply reflect the long-range ordermg. 

These reciprocal lattice vectors, which have units of and are also parallel to the surface, define the LEED pattern in k-space. Each diffraction spot corresponds to the sum of integer multiples of at and at-... 

Figure 7. Continued. B. Diagram of the LEED pattern in A. Continued on next page.

Ordered LEED patterns were evident on all three surfaces. A Cu(100)(/2X/2)R45°-C1 was observed with IEED, the same pattern present on the surface before immersion, but with a slight increase in spot diffuseness. A 28% decrease in Cl Auger current was observed and was responsible for the degradation in LEED pattern clarity. It... 

In fact, the phenomenon and conditions described here can be applied not only to a beam of electrons, but also to a beam of X-rays.12 What is the difference in the diffraction pattern when these different sources of radiation are applied to an ordered array of atoms X-rays penetrate deeply into the ctystal, and information between spacing of planes inside the crystal is obtained from the diffraction pattern. In contrast, the use of low-energy electrons as a source of incident radiation with energies in the range of 10 to 500 eV ensures that only atoms close to the surface (one or two planes) produce the diffraction pattern. Since this is the region in contact with a solution, the region where electrochemical processes occur, LEED is the technique used in electro-... 

In LEED a beam of low-energy electrons rather than x-rays is used to form the diffraction pattern, but otherwise many of the concepts, relationships, and vocabulary are based on x-ray diffraction (Van Hove and Tong 1979). Accordingly, our discussion of LEED includes a review of pertinent aspects of this topic. Since diffractometry can get quite involved, we tailor our review to those subjects most helpful in getting us started and leave more advanced concepts for further study. 

FIG. 9.17 Illustrations of LEED spots and coherent structures formed by adsorbed species, (a) indexing of the LEED spots of W(100) pattern in Figure 9.16a as described in Example 9.7 (b) side view of coherent structures formed by adsorbed (open circles) species on metal surface (c) unit mesh for W(100) with adsorbed oxygen (open circles) (d) unit mesh for W(100) with adsorbed hydrogen (open circles). 

Since the version represented by the solid square in Figure 9.17d best characterizes the adsorption, it is described as a c(2 x 2) net, the c reminding us that this is a centered structure. The full description of the LEED pattern in Figure 9.16c in the Wood notation is therefore written W( 100)—c(2 x 2)—hydrogen. 

The adsorbed CO molecules are under strong lateral repulsion, in particular, adsorption of these molecules on the Ru(001) face leads to an appearance of the R( /3 x /3) ordered structures in LEED patterns [153]. This means absence of an uniform occupation of every adsoprtion site by the CO molecules the CO molecules are accommodated in the different sublattices non-uniformly, and this is the typical case when lateral interactions are responsible for a non-uniform adspecies distribution on a homogenous surface. Splitting of the experimental spectra is a signal of the... 

Fig. 6.9 LEED images of Rh (111) and (100) with ordered layers of ammonia and nitrogen. The insets show the idealized patterns. In the left image, the middle spot in the Rh(111) image is missing because it is covered by the electron gun (see the LEED apparatus in Fig. 6.6). In the second image of Rh(111) with NH3, the sample has been rotated slightly away from the gun, which is visible as the...

Also shown in Figure 6.9 are the LEED patterns of the square Rh(100) and an ordered layer of nitrogen atoms in a c(2 x 2) structure [25]. Actually, c(2 x 2) is a convenient - but incorrect - way to indicate this pattern, based on a unit cell that is larger than needed to describe the structure. The correct unit cell -this is important because it is needed to interpret the diffraction pattern - is (s/2 x /2)R450 with respect to the (1 x 1) unit cell of the substrate, and the right way to describe the structure would be Rh(100) — (s/2 x v )R45° N (which hardly anyone uses). Calculations indicate that the N-atoms occupy the fourfold hollow positions between the rhodium atoms [26], In Chapter 8, we discuss the LEED patterns of CO adsorbed on Rh(lll), which gives two more examples of ordered adsorbate structures and how they appear in LEED experiments (see Fig. 8.15). 

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