Surface specific vibrational spectroscopy

Nonlinear optical infrared-visible sum frequency generation (IR-vis SFG) is a versatile surface-specific vibrational spectroscopy that meets the requirements mentioned above. SFG provides vibrational spectra of molecules adsorbed on a surface, while the molecules in the gas phase do not produce a signal. Consequently, SFG can be operated in a pressure range from UFIV to ambient conditions and still detects only the adsorbed species. A direct comparison of adsorbate structures under UFIV and elevated pressure is therefore feasible. Furthermore, SFG can be applied to molecules adsorbed on single crystals, thin films, metal foils, and supported nanoparticles (46,116-121) and is thus a promising tool to extend surface science experiments to more realistic conditions. 

Both, SPR- and LSPR-based sensors utilize the property that nanostructured thin film or particles are very sensitive to the dielectric constant change of their local environment, and the enhanced detection sensihvity really depends on how the nanostructures respond to such change. Although some of these techniques (especially SPR) are already available commercially, their sensihvity and specificity are relatively poor, especially for trace amounts of biomolecules or complicated systems such as viruses and bacteria. As stated above, metallic nanostructures-especially particles-may also enhance the local electric field when the incident wavelength is close to the LSPRW, and this provides yet another means of designing enhanced spectroscopic sensors, especially the so-called surface-enhanced vibrational spectroscopy (SEVS) sensors. 

Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy. Attenuated total redectance (atr) ftir spectroscopy is based on the principle of total internal redection (40). Methods based on internal redection in the uv and visible regions of the spectmm are also common in addition to those in the ir region. The implementation of internal redection in the ir region of the spectmm provides a means of obtaining ir spectra of surfaces or interfaces, thus providing moleculady-specific vibrational information. 

Hyper-Raman spectroscopy is not a surface-specific technique while SFG vibrational spectroscopy can selectively probe surfaces and interfaces, although both methods are based on the second-order nonlinear process. The vibrational SFG is a combination process of IR absorption and Raman scattering and, hence, only accessible to IR/Raman-active modes, which appear only in non-centrosymmetric molecules. Conversely, the hyper-Raman process does not require such broken centrosymmetry. Energy diagrams for IR, Raman, hyper-Raman, and vibrational SFG processes are summarized in Figure 5.17. 

The primary techniques used in this study include X-ray photoelectron spectroscopy (XPS), reflection-absorption infrared spectroscopy (RAIR), and attenuated total reflectance infrared spectroscopy (ATR). XPS is the most surface-sensitive technique of the three. It provides quantitative information about the elemental composition of near-surface regions (< ca. 50 A sampling depth), but gives the least specific information about chemical structure. RAIR is restricted to the study of thin films on reflective substrates and is ideal for film thicknesses of the order of a few tens of angstroms. As a vibrational spectroscopy, it provides the type of structure-specific information that is difficult to obtain from XPS. The... 

The above discussion is meant to point out specific possible application of surface vibrational spectroscopy to new areas of catalysis. Certainly there are many others and brevity prevents further discussion of such a large subject. Reflection IR, IETS and perhaps Raman, which is rapidly developing in useful directions, would appear to have a good future as high resolution techniques for studies of the chemisorption of organic molecules on a variety of substrates. 

In our discussion the usual Born-Oppenheimer (BO) approximation will be employed. This means that we assume a standard partition of the effective Hamiltonian into an electronic and a nuclear part, as well as the factorization of the solute wavefunction into an electronic and a nuclear component. As will be clear soon, the corresponding electronic problem is the main source of specificities of QM continuum models, due to the nonlinearity of the effective electronic Hamiltonian of the solute. The QM nuclear problem, whose solution gives information on solvent effects on the nuclear structure (geometry) and properties, has less specific aspects, with respect the case of the isolated molecules. In fact, once the proper potential energy surfaces are obtained from the solution of the electronic problem, such a problem can be solved using the standard methods and approximations (mechanical harmonicity, and anharmonicity of various order) used for isolated molecules. The QM nuclear problem is mainly connected with the vibrational properties of the nuclei and the corresponding spectroscopic observables, and it will be considered in more detail in the contributions in the book dedicated to the vibrational spectroscopies (IR/Raman). This contribution will be focused on the QM electronic problem. 

Vibrational sum-frequency spectroscopy (VSFS) is a second-order non-linear optical technique that can directly measure the vibrational spectrum of molecules at an interface. Under the dipole approximation, this second-order non-linear optical technique is uniquely suited to the study of surfaces because it is forbidden in media possessing inversion symmetry. At the interface between two centrosymmetric media there is no inversion centre and sum-frequency generation is allowed. Thus the asynunetric nature of the interface allows a selectivity for interfacial properties at a molecular level that is not inherent in other, linear, surface vibrational spectroscopies such as infrared or Raman spectroscopy. VSFS is related to the more common but optically simpler second harmonic generation process in which both beams are of the same fixed frequency and is also surface-specific. 

To address this gap in understanding, the surface specific nonlinear vibrational spectroscopy, sum frequency generation (SFG) has been applied to study the hexagonal ice surface, Ih. The Ih crystalline form is chosen as the focus of this work because it is the stable form of ice for ambient conditions on Earth. Further, the most abundant exposed face is the hexagonal or basal face. The second most abundant face consists of the cylinder sides, also called the prism face. This work examines both of these common faces. 

VIBRATIONAL SPECTROSCOPY Infrared and Raman spectroscopies have proven to be useful techniques for studying the interactions of ions with surfaces. Direct evidence for inner-sphere surface complex formation of metal and metalloid anions has come from vibrational spectroscopic characterization. Both Raman and Fourier transform infrared (FTIR) spectroscopies are capable of examining ion adsorption in wet systems. Chromate (Hsia et al., 1993) and arsenate (Hsia et al., 1994) were found to adsorb specifically on hydrous iron oxide using FTIR spectroscopy. Raman and FTIR spectroscopic studies of arsenic adsorption indicated inner-sphere surface complexes for arsenate and arsenite on amorphous iron oxide, inner-sphere and outer-sphere surface complexes for arsenite on amorphous iron oxide, and outer-sphere surface complexes for arsenite on amorphous aluminum oxide (Goldberg and Johnston, 2001). These surface configurations were used to constrain the surface complexes in application of the constant capacitance and triple layer models (Goldberg and Johnston, 2001). 

Figure 9.3 clearly shows that solvation of an anion is different from that of a cation an anion accepts H-bonds from H2O molecules, whereas a cation attracts the lone-pair electrons of H2O molecules, thus having the same structural effect as an H-bond donor. This dissymmetry of solvation makes anions and cations behave differently at water surfaces or interfaces, a result that has been experimentally put into evidence by sum-frequency vibrational spectroscopy (38), a surface-specific method described in Ch. 4. Cations thus display tendencies to stay in the bulk, because on arriving at the surface, some of the four H2O molecules bound to them cannot establish H-bonds when arriving on the surface. This is not so for anions, as the H2O bound to them already establish H-bonds on them. The surface therefore favours the presence of anions. This effect is at the origin of the oxidative power of sea water, which is ascribed to Cl anions positioned on surfaces of liquid water droplets. 

Most studies that aim to improve the characterization of the electrode surface have been carried out using high-vacuum techniques such as Auger [1, 2], XPS [3, 4], SIMS [5, 6], etc. However, these techniques involve the removal of the electrode from the electrolyte and the information derived from them may not reflect the state of the electrode in-situ. In addition, many of these techniques lack the molecular specificity afforded by vibrational spectroscopy and it has long been realised that IR spectroscopy would be an ideal method if it could be applied to the in-situ study of the electrode surface. Information from IR would include, potentially, molecular composition and symmetry, bond lengths and force constants (perhaps allowing us to estimate the strength of a chemisorption bond), and molecular orientation. 

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