Site symmetry, adsorbed species

The state of the superoxide ion has been summarized by Naceache et al. 22). It appears probable that an ionic model is most suitable for the adsorbed species since the hyperfine interaction with the adjacent cation is relatively small. Furthermore, the equivalent 170 hyperfine interaction suggests that the ion is adsorbed with its internuclear axis parallel to the plane of the surface and perpendicular to the axis of symmetry of the adsorption site. Hence, the covalent structures suggested by several investigators have not been verified by ESR data. 

In a considerable number of cases both sets of modes have been observed in on-specular VEEL spectra, and the deduction has been made that the symmetry of the surface complex is Cs (or less) (145,146,151,152,160,162). The question remains whether this implies a twofold bridged adsorption site or a neighbor-induced asymmetry within an essentially C3 site, as already described. However, there are examples of species on Pt(lll) (150), Ni(ll 1) (117), and Cu(lll) (161) surfaces for which MSSR as applied to VEEL spectra clearly indicates C3v symmetry of the surface complex, without significant differences in the other frequencies as observed off-specular. These favorable cases may arise from particularly regular arrays of adsorbed species, the presence of which could very profitably be confirmed by LEED. We deduce that the CH3 adsorption sites are intrinsically C3v as far as the bare surface is concerned, i.e., on-top or threefold hollow in nature with the threefold axis of the CH3 group perpendicular to the surface. 

Fig. 5. A summary of the infrared absorption bands exhibited by hydrocarbon ligands on metal atoms in various model compounds. Surface species on metals may give absorptions varying by ca. 50 cm 1 from the band positions in the model-compound spectra in the fingerprint region below 1400 cm. The patterns of band-positions and intensities are significant. M indicates MSSR-allowed modes for an analogous species on a flat surface when the adsorbed species is on a site of high symmetry (--) indicates other absorptions that may occur for adsorption on less symmetrical sites or on small metal particles, vs—very strong s—strong ms—medium strong m—medium mw—medium weak w—weak.

Structurally different adsorption sites are also proposed by Giannantonio et af. They claim that adsorption energies as determined by TPD for the weakly adsorbed species are in agreement with theoretical calculations for the adsorption of H at an atop or bridged position. The adsorption energy for the strongly adsorbed H is in agreement with calculations for adsorption of H at three- and four-fold symmetry surface sites. 

By monitoring the vibration spectra of chemisorbed species as a function of coverage, crystal surface, and temperature, the location and site symmetries of the adsorbed atoms (4-fold, 3-fold, bridge, on-top) can be monitored and variations in site occupancy can be determined. 

When adsorbed molecules are bombarded with electrons, local heating effects occur that lead to thermal desorption. In addition, there is a small but finite probability that electrons in the chemical bonds that hold the adsorbate to the surface will be excited into a repulsive state, leading to the desorption of that molecule either as a neutral species or as a molecular ion. Desorption of neutral species under electron-beam bombardment is frequently observed in studies of electron-surface interactions. A fraction of the adsorbed molecules will be ionized. These can be detected as positive ions, and the spatial distribution of this ion flux can be imaged on a fluorescent screen. Electron-stimulated desorption ion-angular distribution (ESDIAD) [56, 61, 64, 79-84] is the name of the technique that is used to learn about the site symmetry and orientation of adsorbed molecular species, since the molecular ions are usually emitted in the directions of their chemical bonds with the surface and with an unchanged orientation with respect to the orientation of the molecule when it was adsorbed on the surface. 

The structural effects on the extent and nature of the adsorbed species produced during CO2 reduction have been investigated only recently by voltammetric and FTIR studies. The results obtained showed that the formation and bonding of CO-like species is determined by the symmetry of the surface sites. Thus multibonded CO is the main adsorbate formed at a well-ordered Pt(lOO) electrode, while linearly bonded CO was detected on Pt(llO) samples. In the case of the Pt(lll) electrode, no irreversibly adsorbed species are formed at the electrode surface. This behavior of well-ordered Pt(lll) changes significantly when sites with different symmetries were introduced on the surface. 

Complex Molecule Adsorption at Surfaces Determining the basic chemical structure of an adsorbed species is, by far, the largest application of surface IR spectroscopy, utilizing ideas of group frequencies and databases or calculations of fingerprint spectra. Thus, whether the molecule has been adsorbed intact or has been dissociated, whether a preferred molecular conformation has been adopted, and so on are determined. The application of the metal surface dipole selection rule enables the adsorption site symmetry of the adsorbate at a metal surface to be determined and, from this, adsorbate orientation and conformation can be attained. In some... 

These materials have pore sizes comparable to molecular dimensions and allow an approach at the molecular level for understanding of active site action. The electric field strengths and symmetries associated with such structures can induce chemical reactivity in adsorbed species and influence the behavior of surface functional groups (e.g., the acidity of surface silanol groups). 

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