Theory of reflection spectroscopy

Reflection spectroscopy is used for more accurate quantification of the radiation reflected by a sample the intensity, spectral composition, angular distribution and polarisation can be analysed. This method is particularly apt for measuring samples that are impervious to light, that is to say, wherever absorption spectroscopy cannot be used. 

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Reflectance spectroscopy in the infrared and visible ultraviolet regions provides information on electronic states in the interphase. The external reflectance spectroscopy of the pure metal electrode at a variable potential (in the region of the minimal faradaic current) is also termed electroreflectance . Its importance at present is decreased by the fact that no satisfactory theory has so far been developed. The application of reflectance spectroscopy in the ultraviolet and visible regions is based on a study of the electronic spectra of adsorbed substances and oxide films on electrodes. 

A general solution to both problems is the application of attenuated total reflectance (ATR) in combination with infrared spectroscopy. The theory of ATR spectroscopy is well described in several books and articles which also demonstrate the applicability of the Beer-Lambert law to ATR spectroscopy [9]. The combination of reaction calorimetry and ATR spectroscopy is now rather common [ 10-13] typically using commercially available calorimeters. 

Hapke, B. 1993, Theory of Reflectance andEmittance Spectroscopy, Cambridge University Press. 

CW-EPR X-band measurements were performed on a Bruker ESP 300E Spectrometer at a temperature of 120 K. The CuHis complexes are paramagnetic due to the S=l/2 spin of the Cu " ion. Nitrogen physisorption was performed with a Micromeritics ASAP 2400 apparatus. Measurements were done at 77 K. Prior to the measurements the zeolite samples were degassed for 24 hours at 373 K in vacuum. Micropore volumes and pore size distributions were determined with standard BET and BJH theory. Diffuse Reflectance Spectroscopy of the CuHis complex encapsulated zeolite samples were taken on a Varian Cary 5 UV-Vis-NIR spectrophotometer at room temperature. The DRS spectra were recorded against a halon white reflectance standard in... 

B. Hapke, Theory of reflectance and emittance spectroscopy, (Cambridge University Press, Cambridge, 1993). 

The basic optical theory of reflection for a soHd surface covered with a thin film has already been well established. We do not intend to review it here in full detail. Instead, readers can refer to the textbook of optics [12] or review articles on UV-visible reflection spectroscopy at electrode surfaces [2, 6-8]. It must be noted that for some electrode/solution interfaces it is stiU an extremely difficult task to establish the modeling of the interface through the use of classical light reflection theory, as described in the later sections. 

The combination of infrared spectroscopy with the theories of reflection has made advances in surface analysis possible. Attenuated Total Reflectance (ATR) spectroscopy is an innovative technique for proving chemical information of a sample surface and the ability to quantify newly formed species, based upon Pick s second law. The fundamentals of attenuated total reflection (ATR) spectroscopy date back to the initial work of Jacques Fahrenfort and N.J. Harrick, both of whom independently devised the theories of ATR spectroscopy and suggested a wide range of applications. The schematic showing ATR-FTIR configuration is illustrated in Fig. 1 (KS. Kwan, 1998). The penetration depth, d, can be estimated as ... 

Internal reflectance Spectroscopy (IRS) date back to the initial work of Jacques fahrenfort and N.J.Harrrick [1, 2] that independently devised the theories of IRS spectroscopy and suggested a wide range of applications. Internal reflection Spectroscopy is often termed as attenuated total reflection (ATR) spectroscopy. ATR became a popular spectroscopic technique in the early l%0s. 

B. Hapke, Theory of Reflectance and Emittance Spectroscopy, Cambridge Univ. Press,... 

M. W. AT2rs1, Attenuated Total Reflectance Spectroscopy of Polymers Theory and Practice, American Chemical Society, Washington, D.C., 1996. 

In materials investigations surface-sensitive techniques are of special interest. The major contribution of infrared spectroscopy to this field is internal reflection spectroscopy (IRS), often called the "attenuated total reflection" (ATR) technique. To describe theory and principle, electromagnetic wave theory must be apphed [33]. 

The frequencies of absorption bands present gives diagnostic information on the nature of functional groups in materials as well as information from any observed frequency shifts on aspects such as hydrogen bonding and crystallinity. In many cases, spectra can be recorded non-destructively using either reflection or transmission procedures. IR spectra of small samples can also be obtained through microscopes (IR microspectrometry). Chalmers and Dent [8] discuss the theory and practice of IR spectroscopy in their book on industrial analysis with vibrational spectroscopy. Standard spectra of additives for polymeric materials include the major collection by Hummel and Scholl [9]. 

PR. Griffiths and l.M. Olinger, Continnnm theories in diffuse reflection, in Handbook of Vibrational Spectroscopy, l.M. Chalmers and P.R. Griffiths (Eds), Sampling Techniques Vol. 2, John Wiley Sons, Ltd, Chichester, 2002. 

As a rule these methods are based on the concept of evanescent surface waves caused by total internal reflection at a solid/liquid interface. In order to understand the so-called evanescent spectroscopy, a simplified theory of total internal reflection is given below. More complete treatments can be found in various specialized monographs and reviews 145 147>. 

Griffiths, P.R. 8t Olinger, J.M., Continuum Theories in Diffuse Reflection. In Chalmers, J.M. 8c Griffiths, P.R. (eds) Handbook of Vibrational Spectroscopy, Sampling Techniques, Volume 2 John Wiley 8t Sons Chichester, 2002 pp. 1125-1139. 

The empirical modeling element indicates an increased emphasis on data-driven rather than theory-driven modeling of data. This is not to say that appropriate theories and prior chemical knowledge are ignored in chemometrics, but that they are not relied upon completely to model the data. In fact, when one builds a chemometric calibration model for a process analyzer, one is likely to use prior knowledge or theoretical relations of some sort regarding the chemistry of the sample or the physics of the analyzer. For example, in process analytical chemistry (PAC) applications involving absorption spectroscopy, the Beer s Law relation of absorbance vs. concentration is often assumed to be true and in reflectance spectroscopy, the Kubelka-Munk or log(l/P) relations are assumed to be true. 

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