Atomic adsorber

Many surfaces have additional defects other than steps, however, some of which are illustrated in figure A1.7.1(b). For example, steps are usually not flat, i.e. they do not lie along a single low-mdex direction, but instead have kinks. Terraces are also not always perfectly flat, and often contain defects such as adatoms or vacancies. An adatom, is an isolated atom adsorbed on top of a terrace, while a vacancy is an atom or group of atoms missing from an otiierwise perfect terrace. In addition, a group of atoms called an island may fonn on a terrace, as illustrated. 

Elkowitz A B, McCreery J H and Wolken G 1976 Dynamics of atom-adsorbed atom collisions Hydrogen on tungsten Chem. Phys. 17 423... 

Rettner C T 1992 Dynamics of the direct reaction of hydrogen atoms adsorbed on Cu(111) with hydrogen atoms incident from the gas phase Phys.Rev. Lett. 69 383... 

The dangling bonds of a Si surface abstract one F atom from an incident F2 molecule while the complementary F atom is scattered back into the gas phase [20]. This abstractive mechanism leads to F adsorjDtion at single sites rather than at adjacent pairs of sites, as observed directly by scanning tunnelling microscopy [21]. Br atoms adsorb only to Ga atoms in the second layer of GaAs(001)-(2 x 4) where empty dangling bonds on the Ga atoms can be filled by electrons from the Br atoms [22]. 

Sometimes, the system of interest is not the inhnite crystal, but an anomaly in the crystal, such as an extra atom adsorbed in the crystal. In this case, the inhnite symmetry of the crystal is not rigorously correct. The most widely used means for modeling defects is the Mott-Littleton defect method. It is a means for performing an energy minimization in a localized region of the lattice. The method incorporates a continuum description of the polarization for the remainder of the crystal. 

The third case arises when the temperature is higher than 420 K, at which point diffusion of atomic adsorbed oxygen from surface to subsurface region becomes appreciable (103). 

This forms individual hydrogen atoms adsorbed to the surface of the metal. These hydrogen atoms are now available for addition across the alkene. The addition reaction begins when the alkene coordinates with the metal surface ... 

The gas phase partition function Qg s of the atom is the same however, since the atoms are immediately immobilized on a two-dimensional surface, we need to take the configuration of the adsorbed atoms into account in the transition state. Again we consider a surface containing M sites each with an area of a. The density of sites per area is Nq = M/A = 1 /a. The M sites are not necessarily free as some could be occupied already hence, the number of free sites will be M and 0 = M /M = (1-0a)-If we have N atoms adsorbed on these sites (we again use for the transition state Airmobile), the partition function for this system is given by... 

Suppose there are No sites on the surface, then the number of atoms adsorbed is Na... 

A free-electron metal only possesses a broad sp band. Upon approach, the electron levels of the adsorbate broaden and shift down in energy, implying that the adsorbate becomes more stable when adsorbed on the metal. The interaction results in a bonding energy of typically 5 eV for atomic adsorbates on metals. The situation is illustrated in Fig. 6.23. 

Figure 6.24. Interaction between an atomic adsorbate with one valence level and a transition metal, which possesses a broad sp band and a narrow d band located at the Fermi level. The strong interaction with the d band...

Figure 6.28. Schematic illustration of the change in local electronic structure of an oxygen atom adsorbing on the late transition metal rhodium, the DOS of which is shown on the right-hand side. The interaction of the oxygen 2p orbital with the sp band of the transition metal is illustrated through interaction with the idealized free-electron...

The first term is attractive (it increases the bonding energy) while the second is repulsive (decreases the bonding energy). Hence three parameters play a role in determining the bond strength between the metal d band and the atomic adsorbate ... 

Figure 6.29. The matrix element 4d expresses how the d band of the metal couples with the s or p level of the atomic adsorbate, for the three transition metal series. Note how the matrix element increases when movingto the left. It also increases when moving down a column, due to the larger geometrical extent of...

With respect to the adsorption energy, the interaction of the 5cr and 2jt orbitals with the sp band again gives a large and negative (i.e. stabilizing) contribution, E p, to the bond. The hybridization Afj.hyb can be estimated in a similar way to that in the case of atomic adsorbates. 

In the following we consider nitrogen atoms adsorbed on a ruthenium surface that is not completely flat but has an atomic step for each one hundred terrace atoms in a specific direction. The nitrogen atoms bond stronger to the steps than to the terrace sites by 20 kj mok. The vibrational contributions of the adsorbed atoms can be assumed to be equal for the two types of sites. (Is that a good assumption ) Determine how the coverage of the step sites varies with terrace coverage. 

Figure 2 shows the effect of ethylene pressure upon the steady-state reaction rates for clean Ag(llO) at 490 K and 150 torr O2 (25). Also shown is the steady-state coverage of atomically adsorbed oxygen... 

The last term comes from the assumption that every oxygen atom adsorbing on the surface originates from a water molecule of the bulk electrolyte reservoir ... 

In fact, for these two metals (M), adjacent OH groups react spontaneously to yield an H2O molecule and an O atom adsorbed on M. 

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