Orientational structures of adsorbates

Analyzing orientational structures of adsorbates, assume that the molecular centers of mass are rigidly fixed by an adsorption potential to form a two-dimensional lattice, molecular orientations being either unrestricted (in the limit of a weak angular dependence of the adsorption potential) or reduced to several symmetric (equivalent) directions in the absence of lateral interactions. In turn, lateral interactions should be substantially anisotropic. 

Fig. 2.17. Azimuthal angles p, (a) and energies (b) for orientational structures of adsorbed molecules on a square lattice with quadrupole-quadrupole interactions plotted versus the inclination angle 6 reckoned from the surface normal direction. Sublattices j of the structures described are labelled by the numbers 1,2,3, and the boundaries of the orientational phases under consideration are designated by the letters A, B, C, D, E, F, G. Regular and bold lines refer to the limiting cases AUt = 0 and Al/4 kBT(AU4 < 0).

In an effort to understand the mechanisms involved in formation of complex orientational structures of adsorbed molecules and to describe orientational, vibrational, and electronic excitations in systems of this kind, a new approach to solid surface theory has been developed which treats the properties of two-dimensional dipole systems.61,109,121 In adsorbed layers, dipole forces are the main contributors to lateral interactions both of dynamic dipole moments of vibrational or electronic molecular excitations and of static dipole moments (for polar molecules). In the previous chapter, we demonstrated that all the information on lateral interactions within a system is carried by the Fourier components of the dipole-dipole interaction tensors. In this chapter, we consider basic spectral parameters for two-dimensional lattice systems in which the unit cells contain several inequivalent molecules. As seen from Sec. 2.1, such structures are intrinsic in many systems of adsorbed molecules. For the Fourier components in question, the lattice-sublattice relations will be derived which enable, in particular, various parameters of orientational structures on a complex lattice to be expressed in terms of known characteristics of its Bravais sublattices. In the framework of such a treatment, the ground state of the system concerned as well as the infrared-active spectral frequencies of valence dipole vibrations will be elucidated. 

The orientational structure of water near a metal surface has obvious consequences for the electrostatic potential across an interface, since any orientational anisotropy creates an electric field that interacts with the metal electrons. Hydrogen bonds are formed mainly within the adsorbate layer but also between the adsorbate and the second layer. Fig. 3 already shows quite clearly that the requirements of hydrogen bond maximization and minimization of interfacial dipoles lead to preferentially planar orientations. On the metal surface, this behavior is modified because of the anisotropy of the water/metal interactions which favors adsorption with the oxygen end towards the metal phase. 

The crystalline structure of adsorbents directly influences their gas sensitivity. Depending on the type of the problem to be addressed ad-sorption-sensitive semiconductors are monocrystals or monocrystal films with a predetermined crystallographic orientation of of>erational surface vacuum baked polycrystalline films, which, from their electrical standpoint, are similar to monocrystals but differ from the latter by ulti-... 

Fig. 2.14. A schematic representation of orientational structure for adsorbed molecules inclined at the same angle to the surface normal.

To treat the orientational structure of the monolayer formed by 02 molecules on a graphite surface, allowance must be made for the fact that an oxygen molecule is characterized not only by a nonzero magnetic moment but also by a record small quadrupole moment, so that dispersion interactions prevail over quadrupole interactions at intermolecular distances shorter than 10 A.79 In addition, the adsorbate lattice parameters give rise to very small minimum intermolecular distances, a 3.3 A, the parameter b 8.1 A markedly exceeding the values a. That is why, it is sufficient to consider only the nearest-neighbor interactions in a... 

In some cases, like reduction of azulene or for anodic waves, corresponding to mercury salt formations with various ligands, two or even three consecutive adsorption waves can be observed at gradually increased concentration. Two or three adsorbed layers can be formed, which can differ in chemical composition, in number and structure of adsorbed layers, or in orientation of compounds in such layers. 

Figl. Optimized structure of adsorbed CO2 on Ru7 (a) and Co7 (b). Horizontal orientation CO2 in start point of optimization and symmetrical structure of fixed monometallic 7-atomic clusters. 

Similar approach can be used to study phase transitions in films formed on other crystals, e.g., on the (110) faces of fee and bcc crystals of various metals. It is possible to define the appropriate bond-orientational order parameters suitable for determining the formation of registered and uniaxial structures. Such computer simulation studies can be very helpful in determining the role of the surface corrugation on the structure of adsorbed films and the nature of phase transitions between different adsorbed phases. 

Recently we successfully obtained in situ molecular. A.FM images of pyridine base species, pyridine and 5-picoline, adsorbed on cleaved (010) surfaces of natural zeolites. stilbite and heulandite [3-5]. These adsorption systems possessed three adsorption phases one physically adsorbed, and two chemically adsorbed. One of the latter two adsorption phases consists of monolayer of molecules randomly adsorbed, and the other formed a well-ordered (quasi-)hexagonal array. The present paper compares the adsorption characteristics of these adsorption systems in terms of the array and orientation structure of the adsorbed molecules as determined, for the first time, by AFM. 

Recent decades have witnessed spectacular developments in in-situ diffraction and spectroscopic methods in electrochemistry. The synchrotron-based X-ray diffraction technique unraveled the structure of the electrode surface and the structure of adsorbed layers with unprecedented precision. In-situ IR spectroscopy became a powerfiil tool to study the orientation and conformation of adsorbed ions and molecules, to identify products and intermediates of electrode processes, and to investigate the kinetics of fast electrode reactions. UV-visible reflectance spectroscopy and epifluorescence measurements have provided a mass of new molecular-level information about thin organic films at electrode surfaces. Finally, new non-hnear spectroscopies such as second harmonics generation, sum frequency generation, and surface-enhanced Raman spectroscopy introduced unique surface specificity to electrochemical studies. 

NEXAFS is used to determine the orientation of and bonding structure of adsorbates on a surface. For instance, it is used for the analysis of self organizing mono-layers. 

Krilov and Laird [201] have investigated the orientational structure of DHS near two parallel discretely polarized hard walls obtained by fixing point dipoles of equal magnitude and equal angle with the wall normal on a triangular lattice. The dipole-dipole interaction was trimcated at a distance equal to half of the slit width. A strong dependence on is observed for the orientation of the fluid dipoles near the surface. The first adsorbed interfacial layer is found to be almost crystalHzed for the highest dipole moment considered (/z = /2). 

Mielczarski, Suoninen, and their co-workers utilized XPS to provide information on the structure of adsorbed xanthates and dithiophos-phate on metals and metal sulfides. They interpreted their results in terms of the initial distribution of the adsorbed thiol without any special orientation, followed by island formation as the coverage increased and, eventually, a well-ordered monolayer. In subsequent multilayer formation by the metal thiol compound, the orientation of the thiol was considered to become more random. Interestingly, it was noted that other ions preadsorbed on the surface, such as hydroxyl and carbonate, were gradually removed as the xanthate monolayer developed. Only one sulfur environment was observed for surface xanthate species, supporting the conclusion that xanthate is bonded to metal atoms in the surface layer through both sulfur atoms (see Section VII.2). No differences were reported between the binding energies of the thiol in the initial monolayer and those of the bulk thiol compound. 

In order to describe the second-order nonlinear response from the interface of two centrosynnnetric media, the material system may be divided into tlnee regions the interface and the two bulk media. The interface is defined to be the transitional zone where the material properties—such as the electronic structure or molecular orientation of adsorbates—or the electromagnetic fields differ appreciably from the two bulk media. For most systems, this region occurs over a length scale of only a few Angstroms. With respect to the optical radiation, we can thus treat the nonlinearity of the interface as localized to a sheet of polarization. Fonnally, we can describe this sheet by a nonlinear dipole moment per unit area, -P ", which is related to a second-order bulk polarization by hy P - lx, y,r) = y. Flere z is the surface nonnal direction, and the... 

The most common ions observed as a result of electron-stimulated desorption are atomic (e. g., H, 0, E ), but molecular ions such as OH", CO", H20, and 02" can also be found in significant quantities after adsorption of H2O, CO, CO2, etc. Substrate metallic ions have never been observed, which means that ESD is not applicable to surface compositional analysis of solid materials. The most important application of ESD in the angularly resolved form ESDIAD is in determining the structure and mode of adsorption of adsorbed species. This is because the ejection of positive ions in ESD is not isotropic. Instead the ions are desorbed along specific directions only, characterized by the orientation of the molecular bonds that are broken by electron excitation. 

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]. 

In the case of ionic adsorbates, the variation in WS50is normally unable to provide a clue to the molecular structure of the solvent since free charge contributions outweigh dipolar effects. In this case UHV experiments are able to give a much better resolved molecular picture of the situation. The interface is synthesized by adsorbing ions first and solvent molecules afterward. The variation of work function thus provides evidence for the effect of the two components separately and it is possible to see the different orientation of water molecules around an adsorbed ion.58,86,87 Examples are provided in Fig. 6. 

The ordered structure and molecule orientation in the monolayers, as suggested by the Hardy model, have been studied by various means. Electron diffraction techniques, for example, including both reflection and transmission, have been employed to examine the molecular orientation of adsorbed monolayers or surface hlms. The observations from these studies can be summarized as follows [3]. 

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