Substrate adsorbate bond

Vacuum-compatible solids Adsorbate-substrate bond lengths... 

In the investigation of adsorbed species using XAS, it is not always possible to probe adsorbate—substrate bonding by changing to the adsorption edge of the adsorbate, for example, the adsorption of carbon monoxide on carbon supported Pt particles. Carbon... 

Optical excitation of metals with intense femtosecond laser pulses can create extreme non-equilibrium conditions in the solid where the electronic system reaches several thousand degrees Kelvin on a sub-picosecond timescale, while the lattice (phonon) bath, stays fairly cold. As illustrated in Figure 3.22, photoexcited hot electrons may transiently attach to unoccupied adsorbate levels and this change in the electronic structure may induce vibrational motions of the adsorbate-substrate bond. For high excitation densities with femtosecond pulses, multiple excitation/deexcitation cycles can occur and may eventually lead to desorption of adsorbate molecules or reactions with co-adsorbed species. After 1-2 ps, the hot electron... 

From the calculated adsorbate-substrate bond length a substantial covalent character of alkali bonding to transition metal surfaces is deduced in the low coverage limit. A spin-polarized calculation shows that the unpaired spin of the alkali atom is almost completely quenched upon chemisorption. 

Figure 12.12 (a) Idealised single-molecule transport experiment, (b) Single-molecule transport experiment with optimal structural control based on a (1) epitaxial molecular monolayer with well-characterised adsorbate-substrate bond and (2) a specific, chemically well-defined tip-molecule contact, (c) Mechanically gated single-molecule wire based on experiment in b and STM tip retraction. 

Some adsorbates do not form strong chemical bonds with substrate atoms. This situation is called physical adsorption or physisorption. For these adsorbates the adsorbate-adsorbate interactions can dominate the adsorbate-substrate interactions, and the optimum adsorbate-substrate bonding geometry can be overridden by the lateral adsorbate-adsorbate interactions, yielding, for example, incommensurate structures in which the overlayer and the substrate have independent lattices. When... 

The surface atoms move outward, rotate, or are displaced along the surface in order to optimize the strength of the adsorbate-substrate bonds. Thus, the structure of the surface is not rigid but responds to the changing physical-chemical environment at the interface. 

When the adsorbate-substrate bonds are weak (of the van der Waals type), the properties of the adsorbed monolayer are less dependent on the substrate s atomic structure. In this circumstance, two-dimensional adsorbate phases exhibit lattice-gas-liquid-solid transitions as a function of coverage and temperature changes. 

When a single atom or molecule is adsorbed on a clean surface, the heat of adsorption reflects the strength of the adsorbate-substrate bond. As more molecules adsorb, each one contributes its heat of adsorption to the total (integral) measured value. If the heat of adsorption per molecule is the integral heat of adsorption, of... 

Restructuring occurs in order to maximize the bonding and stability of the adsorbate-substrate complex. Thus it is driven by thermodynamic forces and is most likely to occur when the stronger adsorbate-substrate bonds that form compensate for the weakening of bonds between the substrate atoms, an inevitable accompaniment to the chemisorption-induced restructuring process. 

Two macroscopic experimentally determinable parameters characterize the adsorbed monolayer the coverage and the heat of adsorption. The coverage is deflned as the ratio of the number of adsorbed atoms or molecules to the total number of adsorption sites (usually taken as the number of atoms in the surface plane). The heat of adsorption A//ads, is implicitly linked to the strength of the adsorbate-substrate bond. Knowledge of both parameters often reveals the nature of bonding in the adsorbed layer. 

When the adsorbates are present (solid lines), one observes additional peaks due to excitation of the adsorbate-substrate bond, —0.05eV in these systems the internal bond of the adsorbate is not significantly excited at this energy, since it has a considerably larger frequency —0.3eV. The location of each peak is displaced from the expected position, by the average energy (Cph)... 

Adsorbate-substrate bond length The value of the adsorbate-substrate bond length controls the strength of the resonance integrals / . In the case of proton adsorption, this factor cannot discriminate between the various oxides, because the proton-surface bond length remains very close to the 0-H inter-atomic distance in the free hydroxyl group or in... 

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