Stretch, internal, adsorbed

In Fig. 5 we compare HREEL spectra recorded after exposing the flat and stepped Ag surfaces at T = 105 K to small amounts of 02 dosed with E[ = 0.39 eY and at a crystal temperature T = 105 K. The angle of incidence was chosen normal to the crystal for Ag(l 0 0) and nearly normal to the (1 1 0) nanofacets for Ag(4 1 0) and Ag(2 1 0). HREEL spectra indicate that at this temperature only ad-molecules are observed on Ag(l 00), at least for small exposures. This is witnessed in the HREEL spectra by the loss at 81 meV [55], corresponding to the internal stretch motion of adsorbed 02, and by the absence of intensity in the frequency region of the O/Ag stretch, between 30 meV and 40 meV [62]. On Ag(4 1 0) partial dissociation occurs since two Ag/O stretch losses are present, at 32 meV and at 40meV, while the internal 02 vibration is visible at 84meV [96]. On Ag(2 1 0), on the contrary, only the low frequency losses are present, indicating that the admolecules are unstable [97]. Our first conclusion is therefore that open steps cause 02 dissociation and that this mechanism is very effective on Ag(2 1 0) and less efficient on Ag(4 1 0) where the terraces have a finite width. Also in this latter case,... 

The vibrational frequency shift arises from the perturbation of the molecular eigenvalues by adsorption. The interaction potential depends on the internal stretching coordinate =(r-re)/re. The vibrational potential of the adsorbed molecule V(0 is composed of the internal potential of the free molecule and the external potential... 

However, the quantum treatment of these processes increases the role of the energy transfer from the model heat bath to the reaction co-ordinate, resulting in thermal desorption of excited carbon monoxide adsorbates. The other frustrated translational or rotational modes may also play an intermediate role in energy transfer from excited carbon monoxide stretching mode into the adsorption bond mode (CO-surface). These interactions are important because direct coupling between the earbon monoxide internal stretching mode and the frustrated translational mode, responsible for desorption, is very weak. Taking into account these interactions, this leads to drastic acceleration of desorption [5, 49]. 

Fig. 14. An adsorbed molecule shows characteristic vibrational frequencies which provide important information on the adsorption site and on the chemical interaction with the substrate. Whereas the internal vibrations like the internal stretch Vj of the CO-molecule shown in the top can be compared to the corresponding values in the gas-phase, the so-called external vibrations, V2-V4, exist only in for the adsorbate.

With regard to the identification of adsorbed molecular species, vibrational spectroscopy plays a key role. For determining the stoichiometry of a molecule other methods are better suited (e.g. XPS), but the chemical state of an adsorbed molecule can be best identified by vibrational spectroscopy. This is in part due to the fact that a vast amount of data exists for bulk compounds. For example the comparison of C-O stretch frequencies in metal-organic compounds like nickeltetracarbonyl, Ni(CO)4, with corresponding data for the surface species allows important conclusions to be drawn about the nature of the molecular adsorbate. In many cases the number of modes observed in vibrational spectroscopy provides direct information on the symmetry of the adsorption site. It has been found that in many cases the frequency of internal stretching modes shows a correlation with the adsorption site. For example the internal vibration... 

The effects of mass transfer are different in the stationary and mobile phases. The resistance to mass transfer in the mobile phase varies with the reciprocals of mobile phase velocity and the diffusivity of the species. The resistance to mass transfer inside the stationary phase varies with the reciprocal of diffusivity and is proportional to the radius of the adsorbent granules attached to the chromatography plate, or the structural complexity of the internal pores in chromatographic paper. For both types of mass-transfer resistance, band stretching is proportional in each direction, as measured from the geometrical spot center, and increases in magnitude the greater the resistance. 

Upon upd-modification with tin (Fig. 5.75) and nickel (Fig. 5.76), the band assigned to the oxalate-metal stretching mode is shifted only very slightly to 256 cm and to 257 cm respectively. The band position is almost independent of the nature of the upd-metal, implying rather unspecific and weak interactions (physisorption). Based on the evaluation of further internal modes of the adsorbed oxalato anion, small changes in band position indicative of the presence of the upd-metal could be identified. 

Fig. 16 is casted into a simple but quantitative lattice algorithm. The basic idea is that individual chains successively increase their internal order (characterized by the degree of chain folding) during the crystallization process. The more the chain is ordered (the fewer folds it has) the lower is the surface area needed for this chain. The ultimate degree of order is represented by the completely stretched chain which only occupies a surface area proportional to the cross-section of one stem ao (area of a crystalUne unit cell), see Fig. 16. Let Ao be the area of the corresponding liquid chain, flatly adsorbed onto the surface. Then, M = Aq/ao N > 1 chains can occupy the same area Ao in the crystalUne state. By contrast, in a simple growth model [27] the area per particle remains constant and it is the original dilution of particles which is responsible for the various diffusion-controlled patterns [28,55].

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