Polarization dependence surface

Polarization-dependent surface EXAFS measurements have provided some of the best-defined characterizations of adsorbate structures. 

Xu HX, Kail M (2003) Polarization-dependent surface-enhanced Raman spectroscopy of isolated silver nanoaggregates. Chemphyschem 4(9) 1001-1005... 

Viswanathan, N. K., Balasubramanian, S., Li, L., Tripathy, S. K., and Kumar, J. A detailed investigation of the polarization-dependent surface-relief-grating formation process on azo polymer films. Japanese J. Appl. Phys. Part 1—Regular Papers Short Notes Review Papers 1999, 38, pp. 5928-5937. 

The setup for a polarization dependent, surface sensitive soft x-ray absorption experiment is sketched in Fig. 6.4A. The sample is mounted onto a rotatable holder with a vertical rotation axis lying in the sample surface and a second rotation axis along the sample normal. The latter allows rotation of the sample such that the major, horizontal component of the electric field vector rotates within the plane parallel (Fig. 6.3A) or perpendicular (Fig. 6.3B) to the rubbing direction, when the sample is rotated around the vertical rotation axis. 

The nonlinear response of an individual molecule depends on die orientation of the molecule with respect to the polarization of the applied and detected electric fields. The same situation prevails for an ensemble of molecules at an interface. It follows that we may gamer infonnation about molecular orientation at surfaces and interfaces by appropriate measurements of the polarization dependence of the nonlinear response, taken together with a model for the nonlinear response of the relevant molecule in a standard orientation. 

The polarization dependence of the photon absorbance in metal surface systems also brings about the so-called surface selection rule, which states that only vibrational modes with dynamic moments having components perpendicular to the surface plane can be detected by RAIRS [22, 23 and 24]. This rule may in some instances limit the usefidness of the reflection tecluiique for adsorbate identification because of the reduction in the number of modes visible in the IR spectra, but more often becomes an advantage thanks to the simplification of the data. Furthenuore, the relative intensities of different vibrational modes can be used to estimate the orientation of the surface moieties. This has been particularly useful in the study of self-... 

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 Bom-Oppenheimer picture the nuclei move on a potential energy surface (PES) which is a solution to the electronic Schrodinger equation. The PES is independent of the nuclear masses (i.e. it is the same for isotopic molecules), this is not the case when working in the adiabatic approximation since the diagonal correction (and mass polarization) depends on the nuclear masses. Solution of (3.16) for the nuclear wave function leads to energy levels for molecular vibrations (Section 13.1) and rotations, which in turn are the fundamentals for many forms of spectroscopy, such as IR, Raman, microwave etc. 

Because the second harmonic response is sensitive to the polarizability of the interface, it is sensitive to the adsorption and desorption of surface species and is capable of quantifying surface species concentrations. Furthermore, SHG can be used to quantify surface order and determine surface symmetry by measuring the anisotropic polarization dependence of the second harmonic response. SHG can also be used to determine important molecular-level and electrochemical quantities such as molecular orientation and surface charge density. 

We and others have been involved in the study of such systems including Cu/Au(lll),85 86 Ag/Au(lll),87 Pb/Ag(lll),88 and Cu/Pt(lll).89 The first three systems involved the use of epitaxially deposited metal films on mica as electrodes.90 92 Such deposition gives rise to electrodes with well-defined single-crystalline structures. In the last case a bulk platinum single crystal was employed. Because of the single-crystalline nature of the electrodes, polarization dependence studies could be used to ascertain surface structure. 

Fig. 2.8. Left oscillatory part of the reflectivity change of Bi (0001) surface at 8K (open circles). Fit to the double damped harmonic function (solid curve) shows that the Aig and Eg components (broken and dotted curves) are a sine and a cosine functions of time, respectively. Right pump polarization dependence of the amplitudes of coherent Aig and Eg phonons of Bi (0001). Adapted from [25]...

We could not conclude at the moment whether the solvent dependent surface properties are to be explained only by the difference in the depth of graft layer. Another possibility is the change in polar group orientation in graft layer as suggested by Hoffman(18). This arguement will be settled by direct determination of the thickness of graft layer prepared under various conditions. Clarification of the surface layer thickness - sur-... 

The sensitivity of the planes of single crystals to cyclic polarization depends on the kind of the metal and the crystallographic orientation of the plane itself. Metals melting at low temperatures (e.g., Sb, Pb, andZn), with mobile surface atoms, are less prone to surface reconstmction than metals melting at high temperatures (e.g., Pt, Au, Cu, and Ag). This is... 

An important consequence of the presence of the metal surface is the so-called infrared selection rule. If the metal is a good conductor the electric field parallel to the surface is screened out and hence it is only the p-component (normal to the surface) of the external field that is able to excite vibrational modes. In other words, it is only possible to excite a vibrational mode that has a nonvanishing component of its dynamical dipole moment normal to the surface. This has the important implication that one can obtain information by infrared spectroscopy about the orientation of a molecule and definitely decide if a mode has its dynamical dipole moment parallel with the surface (and hence is undetectable in the infrared spectra) or not. This strong polarization dependence must also be considered if one wishes to use Eq. (1) as an independent way of determining ft. It is necessary to put a polarizer in the incident beam and use optically passive components (which means polycrystalline windows and mirror optics) to avoid serious errors. With these precautions we have obtained pretty good agreement for the value of n determined from Eq. (1) and by independent means as will be discussed in section 3.2. 

Information concerning the orientation of the molecule can be inferred from the polarization dependence. NEXAFS is sensitive to bond angles, whereas EXAFS is sensitive to the interatomic distances. Linearly polarized X-rays are best suited for molecules possessing directional bonds. This is best exemplified for flat 7T-conjugated molecules lying flat on surfaces. When the electric field vector E is aligned along the surface normal, features due to the out-of-plane tt orbitals... 

Fig. 38. Substrate dependence of the PS/PMMA domain structure spin-cast from THF a-c on SiOx d-f on octadecyl mercaptane (ODM) [352]. The SFM images have lateral dimensions of 14x14 pm2. a,d as spin cast b after immersion in cyclohexane to remove PS. e after immersion in acetic acid to remove the PMMA-rich phase. The cross-sections (c,f), which were recorded along the lines in (a,b,c,d), reveal the vertical distribution of the PS (dark grey) and PMMA (light grey) phases. The error bar in (c) indicates the accuracy of the superposition procedure. PMMA preferentially adsorbs on the more polar SiOx surface to form a homogeneous layer next to the substrate. On the ODM a PS/PMMA bilayer is observed. Courtesy of U. Steiner...

The layer of solution immediately adjacent to the membrane surface becomes depleted in the permeating solute on the feed side of the membrane and enriched in this component on the permeate side. Equivalent gradients also form for the other component. This concentration polarization reduces the permeating component s concentration difference across the membrane, thereby lowering its flux and the membrane selectivity. The importance of concentration polarization depends on the membrane separation process. Concentration polarization can significantly affect membrane performance in reverse osmosis, but it is usually well controlled in industrial systems. On the other hand, membrane performance in ultrafiltration, electrodialysis, and some pervaporation processes is seriously affected by concentration polarization. 

L. S. O. Johansson, R. I. G. Uhrberg, P. Martensson and G. V. Hansson, Surface-state band structure of the Si(100)2 x 1 surface studied with polarization-dependent angle-resolved photoemission on single domain surfaces, Phys. Rev. B 42, 1305 (1990). 

Chemisorption systems are sometimes used for removing trace concentrations of contaminants, but the difficulty of regeneration makes such systems unsuitable for most process applications so most adsorption processes depend on physical adsorption. The forces of physical adsorption are weaker than the forces of chemisorption so the heats of physical adsorption are lower and the adsorbent is more easily regenerated. Several different types of force are involved. For nonpolar systems the major contribution is generally from dispersion-repulsion (van der Waals) forces, which are a fundamental property of all matter. When the surface is polar, depending on the nature of the sorbate molecule, there may also be important contributions from polarization, dipole, and quadmpole interactions. Selective adsorption of a polar species such as water or a quadrupolar species such as CO2 from a mixture with other nonpolar species can therefore be accomplished by using a polar adsorbent. Indeed, adjustment of surface polarity is one of the main ways of tailoring adsorbent selectivity. 

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