Reconstructed state

The (2x1) structure is also known as missing-row reconstruction because one out of two rows of atoms, aligned along the [110] direction, is missing from the surface layer. We stress also that there are two different realizations of these reconstructed states, in which either the even or the odd rows are missing. 

The interplay between step orientation and surface reconstruction is essential for understanding the formation of the network of steps. Indeed, a step perpendicular to the missing rows cannot zig-zag forming clockwise segments parallel to the missing rows, if expensive domain boundaries between opposite reconstruction states are to be... 

The ideal (001) surface of crystals with the bcc structure is illustrated in fig. 9.16(a). We note that the ideal termination of this surface is a square lattice with a lattice parameter equal to that associated with the bulk crystal itself This geometry may be contrasted with that of the reconstructed geometry in W in fig. 9.16(b). The surface geometry can be continuously altered from the ideal bulk termination to the reconstructed state by imposing a series of surface displacements... 

When a Pt(100) surface is subjected to ultrahigh vacuum conditions, the clean surface spontaneously reconstructs [84] from the (1 x 1) structure to the reconstructed state at room temperature by the adsorption of impurities, such as water molecules to the (5x20) structure [84]. The present (lxl) structure means that the state of the sample treated this way, which gave an LEED pattern of the (100) plane without further treatment under vacuum, has a satisfactory initial crystallographic surface structure from an LEED point of view for the electrochemical experiment. 

The optical properties of a Au(lOO) surface in its reconstructed state differ markedly because of the participation of electronic surface states in the optical excitation these states depend on the crystallographic surface structure [95]. Surface band structure calculations have revealed the existence of empty surface states [96]. These surface states can be shifted in their energy by the electrode potential (Stark shift) [97]. Optical transitions into these states thus become potential dependent. 

Surfaces of Au(llO) and Pt(llO) in u.h.v. reconstruct to 1 x 2 or a mixture of 1 x 2 and 1x3 missing row structures (Fig. 24). In the electrochemical environment, the reconstructed state is stable for a range of electrode potentials smaller than a positive critical value. For potentials more positive than the critical value, the reconstruction is lifted. Potential variation reversibly changes the structure between the reconstructed and unreconstructed states. We will dwell on the missing row reconstruction in the following text, the easiest example for tutorial purposes, since it was rationalized by a relatively simple theory. 

Under the influence of the adsorbate, the surface structure may switch periodically between more (=non-reconstructed) and less reactive (=reconstructed) state, whereby the driving forces are the difference in surface free energy of the clean planes on the one hand and the difference in CO adsorption energy on the other. In other words, the reconstructed phase adsorbs CO more rapidly than it is reacted. Thus, the CO coverage increases beyond its critical value for nucleation of the structural transformation into the non-reconstructed state. The latter exhibits an increased oxygen coefficient so that CO is removed more rapidly from the surface. As a result, the CO coverage drops and the surface transforms back to reconstructed states. 

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