Spectroscopic surface charge density

B. Ehrenberg, Spectroscopic methods for the determination of membrane surface charge density, Methods Enzymol. 127, 678-696 (1986). 

Several methods have been developed to calculate the surface electronic structure self-consistently for transition metal systems. All of these involve modeling the surfaces by thin slabs (or by repeated slabs in the case of the supercell approach) and expanding the electron wavefunctions in some basis sets. In conjunction with pseudopotentials, the mixed basis or the LCAO basis are most commonly employed. With basically the surface geometry as input, these calculations yield the work function, surface states, adsorbate states, surface charge densities, densities of states, and often information on preferred sites of adsorption. Surface states are shown to be important in the interpretation of spectroscopic measurements, and chemisorption studies give valuable information concerning the nature of the surface chemical bond. 

Eisenthal and coworkers have employed second-harmonic generation as a spectroscopic tool to characterize surface charge transfer complexes. In a case study, catechol was anchored to 0.4 jxm Ti02 colloidal particles at sufficiently low particle densities, to... 

Some very important surface properties of solids can be properly characterized only by certain wet chemical techniques, some of which are currently under rapid improvement. Studies of adsorption from solution allow determination of the surface density of adsorbing sites, and the characterization of the surface forces involved (the energy of dispersion forces, the strength of acidic or basic sites and the surface density of coul-ombic charge). Adsorption studies can now be extended with some newer spectroscopic tools (Fourier-transform infra-red spectroscopy, laser Raman spectroscopy, and solid NMR spectroscopy), as well as convenient modern versions of older techniques (Doppler electrophoresis, flow microcalorimetry, and automated ellipsometry). 

It has been emphasized that STM is sensitive to topography convoluted with the electronic density of states. Spectroscopic characterization of surface states by STM is a challening field of research to be intensified for a better understanding of the chemical reactivity of interfaces. There are still fundamental effects which could be clarified definitively by direct observation. The characterization of transport properties, as demonstrated in Sec. 6, is complementary to STM and STS, and the combination of several techniques should provide a comprehensive description of charge transfer at electrodes. 

The use of optical methods which probe interface electronic and vibrational resonances offers significant advantages over conventional surface spectroscopic methods in which, e.g. beams of charged particles are used as a probe, or charged particles emitted from the surface/interface after photon absorption are detected. Recently, three-wave mixing techniques such as second-harmonic generation (SHG) have become important tools to study reaction processes at interfaces. SHG is potentially surface-sensitive at nondestructive power densities, and its application is not restricted to ultrahigh vacuum (UHV) conditions.However, SHG suffers from a serious drawback, namely from its lack of molecular selectivity. As a consequence, SHG cannot be used for the identification of unknown surface-species. 

The goal in applying any SCM is to develop a self-consistent methodology for parameter estimation such that a set of standard parameters to describe surface acidity, site density, and the charge/potential relationships for different minerals can be developed and can be used in conjunction with spectroscopic data to guide the selection of appropriate adsorption reactions for the formation of metal ion surface complexes (i.e., inner vs. outer sphere, mono vs. hidentate, mononuclear vs. 

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