Adsorbate symmetries

The coincidence of substrate and adsorbate symmetries is crucial for the ordering, i.e., for the formation of 2D crystals. Additionally, the HBC 22 molecule, which itself resembles a graphite segment, is oriented along the HOPG lattice in the same way as an (second) imposed graphite layer.230 This concludes that the substrate rules the orientation of the molecules. Ordered monolayers of HBC could also be used as a ground floor for deposition of a second layer... 

Measurement of the rotationally anisotropic second harmonic yield has been shown to be a versatile tool for a nonintrusive, relatively simple determination of interface symmetry. Besides plain surfaces it, of course, has also been applied to the determination of adsorbate symmetries, which turns out to be a difficult task in the case of adsorption on insulating surfaces. For example, the adsorption symmetry of molecular water on alkaline-earth halides such as CaF2 has been deduced as a function of coverage (Zink et al. 1992), demonstrating oriented initial adsorption. This orientation is subsequently lost for higher coverages. 

An interesting point is that infrared absorptions that are symmetry-forbidden and hence that do not appear in the spectrum of the gaseous molecule may appear when that molecule is adsorbed. Thus Sheppard and Yates [74] found that normally forbidden bands could be detected in the case of methane and hydrogen adsorbed on glass this meant that there was a decrease in molecular symmetry. In the case of the methane, it appeared from the band shapes that some reduction in rotational degrees of freedom had occurred. Figure XVII-16 shows the IR spectrum for a physisorbed H2 system, and Refs. 69 and 75 give the IR spectra for adsorbed N2 (on Ni) and O2 (in a zeolite), respectively. 

Infrared Spectroscopy. The infrared spectroscopy of adsorbates has been studied for many years, especially for chemisorbed species (see Section XVIII-2C). In the case of physisorption, where the molecule remains intact, one is interested in how the molecular symmetry is altered on adsorption. Perhaps the conceptually simplest case is that of H2 on NaCl(lOO). Being homo-polar, Ha by itself has no allowed vibrational absorption (except for some weak collision-induced transitions) but when adsorbed, the reduced symmetry allows a vibrational spectrum to be observed. Fig. XVII-16 shows the infrared spectrum at 30 K for various degrees of monolayer coverage [96] (the adsorption is Langmuirian with half-coverage at about 10 atm). The bands labeled sf are for transitions of H2 on a smooth face and are from the 7 = 0 and J = 1 rotational states Q /fR) is assigned as a combination band. The bands labeled... 

Because of the generality of the symmetry principle that underlies the nonlinear optical spectroscopy of surfaces and interfaces, the approach has found application to a remarkably wide range of material systems. These include not only the conventional case of solid surfaces in ultrahigh vacuum, but also gas/solid, liquid/solid, gas/liquid and liquid/liquid interfaces. The infonnation attainable from the measurements ranges from adsorbate coverage and orientation to interface vibrational and electronic spectroscopy to surface dynamics on the femtosecond time scale. 

Pemberton J E, Bryant M A, Sobooinski R L and Joa S L 1992 A simple method for determination of orientation of adsorbed organios of low symmetry using surfaoe-enhanoed Raman soattering J. Rhys. Chem. 96 3776-82... 

Figure C3.2.19. In this ESDIAD experiment where ions are produced and collected (see text), an adsorbed acetate species is excited by an incoming electron. ions are emitted in tire direction of tire C-H bond in tire upward pointing -CH group in tire species. Circular symmetry of figure indicates tliat C-H bonds are spinning around tire vertical axis in tire acetate species. From Lee J G, Aimer J, Mocutta D, Denev S and dates J T Jr 2000 J. Chem. Phys. 112 335.

Sometimes, the system of interest is not the inhnite crystal, but an anomaly in the crystal, such as an extra atom adsorbed in the crystal. In this case, the inhnite symmetry of the crystal is not rigorously correct. The most widely used means for modeling defects is the Mott-Littleton defect method. It is a means for performing an energy minimization in a localized region of the lattice. The method incorporates a continuum description of the polarization for the remainder of the crystal. 

The lack of a well-defined specular direction for polycrystalline metal samples decreases the signal levels by 10 —10, and restricts the symmetry information on adsorbates, but many studies using these substrates have proven useful for identifying adsorbates. Charging, beam broadening, and the high probability for excitation of phonon modes of the substrate relative to modes of the adsorbate make it more difficult to carry out adsorption studies on nonmetallic materials. But, this has been done previously for a number of metal oxides and compounds, and also semicon-... 

Surface SHG [4.307] produces frequency-doubled radiation from a single pulsed laser beam. Intensity, polarization dependence, and rotational anisotropy of the SHG provide information about the surface concentration and orientation of adsorbed molecules and on the symmetry of surface structures. SHG has been successfully used for analysis of adsorption kinetics and ordering effects at surfaces and interfaces, reconstruction of solid surfaces and other surface phase transitions, and potential-induced phenomena at electrode surfaces. For example, orientation measurements were used to probe the intermolecular structure at air-methanol, air-water, and alkane-water interfaces and within mono- and multilayer molecular films. Time-resolved investigations have revealed the orientational dynamics at liquid-liquid, liquid-solid, liquid-air, and air-solid interfaces [4.307]. 

The singlet-level theory has also been used to describe the structure of associating fluids near crystalline surfaces [30,31,76,77]. The surface consists explicitly of atoms which are arranged on a lattice of a given symmetry. The fluid atom-surface atom potential can also involve an associative term, i.e., the chemical-type bonding of the adsorbate particles with the surface may be included into the model. However, we restrict ourselves to the case of a nonassociative crystalline surface first. 

It is natural to consider the case when the surface affinity h to adsorb or desorb ions remains unchanged when charging the wall but other cases could be considered as well. In Fig. 13 the differential capacitance C is plotted as a function of a for several values of h. The curves display a maximum for non-positive values of h and a flat minimum for positive values of h. At the pzc the value of the Gouy-Chapman theory and that for h = 0 coincide and the same symmetry argument as in the previous section for the totally symmetric local interaction can be used to rationalize this result. 

Earlier in this review, the relationship between the Raman and infrared spectra of molecules possessing high or low symmetry was considered. It was indicated that for molecules possessing a center of symmetry, no vibration is active in both the Raman and infrared spectra. Several adsorbates in this category and one of intermediate symmetry have been studied by laser Raman spectroscopy (Table IX), and most of these spectra are considered in this section. 

Type Adsorbate Adsorbent Possible symmetry change X-> Y Predicted coincidences1 X Y Changes observed Ref. 

The symmetry listed for Y is for a strong adsorbate-adsorbent interaction. The symmetry may vary with different types of adsorption. 

Changes in relative peak intensity and marginal line shifts have been observed for benzene adsorbed on porous glass (26). More significantly, infrared spectroscopic evidence had been found in the appearance of inactive fundamentals for the lowering of molecular symmetry of benzene on adsorption on zeolites (47). 

Raman spectra, 296, 298, 303, 304 of adsorbed molecules, 333-339 of adsorption systems, 320-332 of Cab-O-Sil disk, 320 different from infrared spectra, 302-304 effect of fluorescence on, 321-327 molecular symmetry and, 304, 305 of oxides, 321... 

Gas phase experiments show that the reactivity of small clusters varies greatly with cluster size (H), and experimental work with small clusters on supports has shown (12) that adsorbed molecules or atoms affect the metal-metal bond length. In both experiments the adsorption site is not known. Also the reactions proceed to completion so that only the high coverage case has been measured. We have considered the chemisorption of several atoms on Be 13 and AI13 clusters. For both clusters trigonal symmetry is maintained, and only the metal-metal... 

The three-dimensional symmetry is broken at the surface, but if one describes the system by a slab of 3-5 layers of atoms separated by 3-5 layers of vacuum, the periodicity has been reestablished. Adsorbed species are placed in the unit cell, which can exist of 3x3 or 4x4 metal atoms. The entire construction is repeated in three dimensions. By this trick one can again use the computational methods of solid-state physics. The slab must be thick enough that the energies calculated converge and the vertical distance between the slabs must be large enough to prevent interaction. 

Although this type of reaction is symmetry forbidden in an unadsorbed molecule, theoretical calculations showed that in a molecule adsorbed on transition metals, such a shift is allowed [3-5], Later, other theoretical calculations suggested another type of 1,3-hydrogen shift, one in which the allylic cxo-hydrogen is abstracted by the surface fi-om an adsorbed alkene (either 1,2-diadsorbed or n-complexed) and the resulting 7i-allyl species moves over the abstracted hydrogen in such a way that it adds to the former vinylic position and causes, in effect, a stepwise intramolecular 1,3-hydrogen shift (bottom shift) [6],... 

The symmetry of the anions and the surface stmcture tetrahedral bisulfates bond very strongly to threefold symmetry sites of (111), but weakly to (100) and (110) surfaces. Halides adsorb more strongly on (100) surfaces. 

Doyen [158] was one who theoretically examined the reflection of metastable atoms from a solid surface within the framework of a quantum- mechanical model based on the general properties of the solid body symmetry. From the author s viewpoint the probability of metastable atom reflection should be negligibly small, regardless of the chemical nature of the surface involved. However, presence of defects and inhomogeneities of a surface formed by adsorbed layers should lead to an abrupt increase in the reflection coefficient, so that its value can approach the relevant gaseous phase parameter on a very inhomogeneous surface. 

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