Wood s notation

Adsorption of 0.05 monolayers (ML) of CO on this surface gives rise to a peak at 2015 cm-1 corresponding to the internal C-0 stretch frequency of the molecule in the on-top adsorption site and one at 470 cm-1 due to the metal-molecule bond. The latter is not easily observable in infrared spectroscopy. Increasing the CO coverage to 0.33 ML enhances the intensity of the HREELS peaks. In addition, the C-O stretch frequency shifts upward because of dipole-dipole coupling [16, 17]. The LEED pattern corresponds to an ordered (V3xV3)R30° overlayer in Wood s notation (see the Appendix) in accordance with the coverage of 0.33 ML. 

Figure A.3 The structure of a few common ordered overlayers and the corresponding Wood s notation.

Figure 6.9 illustrates LEED patterns of the clean Rh(lll) surface, and the surface after adsorption of 0.25 monolayer of NH3 [23], The latter orders in the primitive (2 x 2) overlayer structure (see Appendix for the Wood s notation). In... 

Adsorbates may form ordered overlayers, which can have their own periodicity. The adsorbate structure is given with respect to that of the substrate metal. For simple arrangements the Wood s notation is used, and some examples are given in Figure A.3. The notation Pt(110)-c(2 x 2)0 means that oxygen atoms form an... 

Construction of the (31/2 x 31/2) R 30° superlattice of the (111) plane of an FCC crystal. If this is Au, then the full Wood s notation designation is Au(111) (31/2x31/2) R 30°. Shown are an acute surface unit cell (center) and the preferred obtuse unit cell (lower left) with basis vectors a-m = a/2+6/2 and = b/2 + c/2, with lengths 2 1/2a and 2 1/2a each and with an included angle of 120°. Also shown are two alternate but equivalent settings of the primitive (31/2 x 31/2) R 30° supercell The bottom one is anchored at interstitial sites (shaded circles) with basis vectors... 

System notation Wood s notation dimensions (A) (A) alignment (A) substrate... 

Two sets of notation are commonly used to describe overlayer structures observed in diffraction experiments, the Wood notation [92] and a matrix notation. Although the latter is more flexible, the former is more widely used and we shall restrict ourselves to it in this review. The nomenclature is based on a comparison between the unit mesh of the topmost layer, the overlayer, and that of the second, unreconstructed, substrate layer. If a and b are the unit mesh vectors of the substrate layer and a, and bg the unit mesh vectors of the overlayer, then Wood s notation for an overlayer of adsorbed species A on the hkl plane of a crystal M is... 

In both cases, the adsorbate occupies the T positions with half coverage and the unit cell is then doubled in the h and k directions so the Wood s notation for this... 

The figure tells that both the pure (i.e., antisite-free) A1 termination and the phase separation (with the first and third layers consisting only of antisite atoms) are stable phases. Besides, only two stable superstructures - in which the A1 atoms together with Co antisites in the top layer exhibit chemical order-are found. They are displayed in Figure 11.17. In the (4 x 4)diag superstmcture (Wood s notation (2-s/2 x -s/2)R45°), diagonal rows of antisites reside in the top layer with fourfold vertical and horizontal spacings in between and below the top layer there is unperturbed B2 order. The other stable superstructure, c(2 x 2)+, owns 50% antisites in the top layer and, additionally, the third layer fully consists of antisites. 

Surface reconstructions are herein referred to by Wood s notation (N x M) R(p. N and M refer to the length of the lattice vectors for the surface reconstruction, where the vectors are N and M times the length of the bulk lattice vectors in the plane of the surface. R

surface lattice vectors N and M are rotated by q> degrees with respect to the bulk lattice vectors in the same plane. When R(p is omitted, the angle q> is 0°. Other symmetry indicators can also be included, for example the SrTiOa (100) c(4 x 2) reconstruction has lattice vectors 4 and 2 times the (010) and (001) bulk lattice vectors, in the same direction (0° rotation), while the c indicates that it is a centered unit cell. 

---