Ordering of adsorbates

In addition to the surface/interface selectivity, IR-Visible SFG spectroscopy provides a number of attractive features since it is a coherent process (i) Detection efficiency is very high because the angle of emission of SFG light is strictly determined by the momentum conservation of the two incident beams, together with the fact that SFG can be detected by a photomultiplier (PMT) or CCD, which are the most efficient light detectors, because the SFG beam is in the visible region, (ii) The polarization feature that NLO intrinsically provides enables us to obtain information about a conformational and lateral order of adsorbed molecules on a flat surface, which cannot be obtained by traditional vibrational spectroscopy [29-32]. (iii) A pump and SFG probe measurement can be used for an ultra-fast dynamics study with a time-resolution determined by the incident laser pulses [33-37]. (iv) As a photon-in/photon-out method, SFG is applicable to essentially any system as long as one side of the interface is optically transparent. 

In the microscopic techniques discussed above, the challenge was to visualize the atomic detail. However, in catalysis one also encounters phenomena that occur on the scale of micrometers or millimeters which ask for imaging. In particular, the ordering of adsorbates in large islands and the development of spatio-temporal patterns in oscillating reactions [8], This spectacular phenomenon has stimulated the exploration of imaging techniques that provide information on patterns on the micrometer to millimeter scale. 

Figure 7.23 Ordering of adsorbates on a surface into islands gives rise to regions of different work function, which can be imaged because of the associated differences in photoelectron intensity. The principle forms the basis of photoemission electron microscopy (PEEM). The same principle underlies the imaging of single molecules in the field electron microscope (FEM) (see also Fig. 7.9).

STM has also been used to examine the dynamics of potential-dependent ordering of adsorbed molecules [475-478]. For example, the reversible, charge-induced order-disorder transition of a 2-2 bipyridine mono-layer on Au(lll) has been studied [477]. At positive charges, the planar molecule stands vertically on the surface forming polymeric chains. The chains are randomly oriented at low surface charge but at higher potentials organize into a parallel array of chains, which follow the threefold symmetry of the Au(l 11) substrate as shown in Fig. 34. Similar results were found for uracil adsorption on Au(lll) and Au(lOO) [475,476]. 

This process of separation, also known as column chromatography, was first developed in 1900by Day, an American petroleum chemist. However, a more extensive study was made in 1906 by Tswett, a Polish botanist. He observed that when a solution of plant pigments in petroleum ether is passed slowly through a column packed with alumina, a number of horizontal bands of different colours are produced in the column. This is, evidently, due to the fact that different constituents of the mixture are adsorbed to different extents. The most readily adsorbed constituent is held at the top. The others with decreasing order of adsorbabilities are held up in different zones down the column in the same order. This, of course, gives only a partial separation of the various constituents as some of the less readily adsorbed... 

Note, however, that kinetic descriptions as discussed here represent a serious simplification with respect to ignoring the possibility of lateral interactions. Ordering of adsorbates, even to the extent that reactants organize themselves in islands of substantial dimensions, has a profound influence on the kinetics. Exploration of these effects through Monte Carlo simulations is a field of growing importance [54,55]. 

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