Surface interaction energetics

Although still used the Langmuir equation is only of limited value since in practice surfaces are energetic inhomogeneous and interactions between adsorbed species often occur. 

Energetic particles interacting can also modify the structure and/or stimulate chemical processes on a surface. Absorbed particles excite electronic and/or vibrational (phonon) states in the near-surface region. Some surface scientists investigate the fiindamental details of particle-surface interactions, while others are concerned about monitormg the changes to the surface induced by such interactions. Because of the importance of these interactions, the physics involved in both surface analysis and surface modification are discussed in this section. 

Of course the real projectile-surface interaction potential is not infinitely hard (cf figure A3,9,2. As E increases, the projectile can penetrate deeper into the surface, so that at its turning point (where it momentarily stops before reversing direction to return to the gas phase), an energetic projectile interacts with fewer surface atoms, thus making the effective cube mass smaller. Thus, we expect bE/E to increase with E (and also with W since the well accelerates the projectile towards the surface). 

Atom-surface interactions are intrinsically many-body problems which are known to have no analytical solutions. Due to the shorter de Broglie wavelengdi of an energetic ion than solid interatomic spacings, the energetic atom-surface interaction problem can be treated by classical mechanics. In the classical mechanical... 

At low adsorbate loadings, the differential heat of adsorption decreases with increasing adsorbate loadings. This is direct evidence that the adsorbent surface is energetically heterogeneous, ie, some adsorption sites interact more strongly with the adsorbate molecules. These sites are filled first so that adsorption of additional molecules involves progressively lower heats of adsorption. 

Initial reconstruction caused by flame armealing is stopped when the surface is cooled in the atmosphere, though not in water. The rate of transition from unreconstructed to reconstructed surface is determined by the height of the activation barrier [348], especially at the room temperature. Reconstruction may be removed by adsorption of atoms and molecules [349], since unreconstructed, and thus, more open surface, interacts with the adsorbates stronger than does the densely packed surface. Therefore, the removal of reconstructed surface proceeds from the less to the more energetically favored state [348]. Reconstruction coupled with the formation of more dense surface structure may lead to quite a strong increase in the number of surface atoms. For instance, the Au(100)-(1 X 1) Au(100)-(hex) reconstruction is accompanied by the increase in the number of surface atoms by 24%. 

Emphasis will also be placed on approaches which lead to the removal of reactive species from the gas phase as well as the special role of energetic positive ions in plasma-surface interactions. Controlled scavenging of critical species from the gas phase and/or at specific surfaces and the degree of positive ion bombardment at a given surface can in fact result in simultaneous polymerizatidn at one surface and etching at another within the same apparatus. 

In order to get a more realistic description of surface reactions energetic interactions must be taken into account. We introduced in Section 9.2.1 a general model which is able to handle systems which include mono- and bimolecular steps like adsorption, desorption, diffusion and reaction [38]. Here we apply this model to an extended version of the ZGB-model which incorporates particle diffusion and desorption [41]. 

Ligand-protein contact (LPC) data describe putative surface interactions between the ligand and the site residues and predict whether they are energetically favourable or unfavourable [18,19], Putative hydrogen bonds are labelled as backbone, functional group donors or acceptors or amphiprotic. Putative hydrogen bonds with bond lengths less than or equal to 3.5A are accepted and considered in this analysis. 

The main goal of chemical modification of the surface is to create a preferably uniform surface with a selected type of interactions. Energetic uniformity of the surface is dependent on the ligand s bonding density, distribution, and conformations. An additional desirable feature is hydrolytic stability, which in most cases is achieved by proper shielding of anchoring bonds. 

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