Stretch, internal, adsorbed molecules

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... 

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... 

CF4 adsorbs on closed SWCNTs, exhibiting its V3 asynunetric stretching mode at 1267 cm (red shift relative to the gas phase, 15 cm ) [94]. When adsorption occurs in the interior of opened SWCNTs, internally bound CF4 exhibits its V3 mode at 1247 cm (red shift relative to the gas phase, 35 cm ). Qualitatively similar red shifts were seen in the case of adsorption of NO on SWCNTs [94]. Upon NO adsorption, the molecules dimerized, forming (NO)2, with the asymmetric stretch vibration for the interior-adsorbed dimer occurring at 1257 cm , while the gas phase frequency for this vibration is known to be 1289 cm . The vibrational frequency of the Si—H bond in octasilox-ane, Si8HgOi2 (a cube-shaped molecule), normally appears as a sharp band at 2277 cm in solution. Upon insertion in CNTs, the Si—H absorption band is... 

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