Optical constants surface states

Greenler approached the problem of the calculation of the absorption A by describing the absorbing material on the surface as a dielectric with optical constants approaching the values of the adsorbate in the solid or liquid state. The calculation of A according to Greenler [20] requires solution of a set of relatively complicated expressions to evaluate the electric field at the surface. More simplified treatments... 

The change of state of polarization caused by the reflection is described by two quantities, which are both angles A, a phase change term, and f, the tangent of which describes the amplitude change. For a clean, bare surface, these quantities are related to the optical constants n and k of the substrate in a one-to-one mapping. Ellipsometry is therefore one of the principal ways of evaluating the optical constants of materials. 

As Fig.l shows, one beam of linearly polarized light with known wavelength, incident onto the surface of an isotropy sample, the polarization state of the incident light turns to elliptical polarization. This variation in polarization state depends on the parameters that related to the sample and light, such as the incident angle of light, the refractive index of the sample. If the sample is multilayered thin film system, the parameters will include the optical constants and thickness of each layer, etc. 

When light is reflected at the surface separating two homogeneous, isotropic, semi-infinite phases, each one being characterized by its optical constants, (wj, fcj) and (fi2, 2), respectively, both the intensity and the state of polarization are affected. 

The molecular constants that describe the stnicture of a molecule can be measured using many optical teclmiques described in section A3.5.1 as long as the resolution is sufficient to separate the rovibrational states [110. 111 and 112]. Absorption spectroscopy is difficult with ions in the gas phase, hence many ion species have been first studied by matrix isolation methods [113], in which the IR spectrum is observed for ions trapped witliin a frozen noble gas on a liquid-helium cooled surface. The measured frequencies may be shifted as much as 1 % from gas phase values because of the weak interaction witli the matrix. 

Our intention is to give a brief survey of advanced theoretical methods used to detennine the electronic and geometric stmcture of solids and surfaces. The electronic stmcture encompasses the energies and wavefunctions (and other properties derived from them) of the electronic states in solids, while the geometric stmcture refers to the equilibrium atomic positions. Quantities that can be derived from the electronic stmcture calculations include the electronic (electron energies, charge densities), vibrational (phonon spectra), stmctiiral (lattice constants, equilibrium stmctiires), mechanical (bulk moduli, elastic constants) and optical (absorption, transmission) properties of crystals. We will also report on teclmiques used to study solid surfaces, with particular examples drawn from chemisorption on transition metal surfaces. 

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