Reflection-absorption spectroscopy principles

Mendelsohn, R., Mao, G., Flach, C.R., 2010. Infrared reflection—absorption spectroscopy principles and applications to lipid—protein interaction in Langmuir films. Biochim. Biophys. Acta 1798, 788—800. 

Electrochemists have used IR spectroscopy for many years to probe electrodeelectrolyte interfaces 107). The most popular technique is IR reflection absorption spectroscopy (IRRAS) 108). A schematic comparison of the principle of ATR and IRRAS experiments is shown in Fig. 37. One advantage of the ATR over the IRRAS technique for catalytic applications concerns diffusion. In IRRAS experiments, the IR beam passes through a thin liquid film between a window and the sample. This... 

A second lock-in detection method that was employed is polarization modulation, which involves modulating the polarization state of the incident infrared beam, and is again an extension of an approach developed for the study of the gas-solid interface [80]. Polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) relies upon the principles underlying the surface selection rule... 

The infrared spectroscopy has been used largely in Surface Science to study adsorbed molecules on surfaces in vacuum. The infrared beam is reflected at the crystal surface and the adsorbed molecules interact with the incident light [1, 2). Greenler proposed the basic principles of the reflection-absorption spectroscopy in 1966 [3]. From his work, it becomes clear that the light interacting with the adsorbed molecules on a metal surface is only the parallel mode of the electric field of the incident radiation. This leads to the sosurface selection rule, a central concept in reflection-absorption spectroscopy (see Sect. 3.4.2.1.2). 

Since dp is at most a few micrometers in the IR region, an ATR spectrum provides information regarding the sample s surface. Thus, the ATR method is especially applicable to the case of thin macromolecular materials such as fibers and fabrics, and also to thin polymer layers (especially coatings and laminates) which often fail to yield useful transmission spectra. Other sophisticated methods that are useful for examining thin polymer layers include infrared ellipsometry IRE) and infrared reflection absorption spectroscopy IRRAS) the principles of these techniques are briefly described in Section 2.2.5.1. 

IR absorption spectroscopy follows the principles of classical absorption spectrocopy. A sample is irradiated with electromagnetic radiation and the transmitted or reflected radiation is (spectrally) analysed. In IR spectroscopy, the wavelengths used are in the pm-range. 

Summarizing, infrared spectroscopy measures, in principle, force constants of chemical bonds. It is a powerful tool in the identification of adsorbed species and their bonding mode. Infrared spectroscopy is an in situ technique, which is applicable in transmission or diffuse reflection mode on real catalysts, and in reflection-absorption mode on single crystal surfaces. Sum frequency generation is a speciality... 

Principles and Characteristics Absorption spectroscopy of both vapour and liquid samples by wavelengths in the UV/VIS range, causing electronic transitions in the sample, can be used to quantify components in a mixture. Optical transmission measurements are preferred to diffuse reflectance, they provide higher sensitivity, more precision and enable monofilament fibre optics to be used. Spectroscopic (UVA IS/NIR) analysis of pellets is more complicated. 

The detailed examination of the behavior of light passing through or reflected by an interface can, in principle, allow the determination of the monolayer thickness, its index of refiraction and absorption coefficient as a function of wavelength. The subjects of ellipsometry, spectroscopy, and x-ray reflection deal with this goal we sketch these techniques here. 

In the preceding section, we presented principles of spectroscopy over the entire electromagnetic spectrum. The most important spectroscopic methods are those in the visible spectral region where food colorants can be perceived by the human eye. Human perception and the physical analysis of food colorants operate differently. The human perception with which we shall deal in Section 1.5 is difficult to normalize. However, the intention to standardize human color perception based on the abilities of most individuals led to a variety of protocols that regulate in detail how, with physical methods, human color perception can be simulated. In any case, a sophisticated instrumental set up is required. We present certain details related to optical spectroscopy here. For practical purposes, one must discriminate between measurements in the absorbance mode and those in the reflection mode. The latter mode is more important for direct measurement of colorants in food samples. To characterize pure or extracted food colorants the absorption mode should be used. 

The signal generation principle of Raman is inelastic molecular light scattering, in contrast to resonant energy absorption/emission in IR spectroscopy. During the measurement, the sample is irradiated with intense monochromatic radiation. While most of this radiation is transmitted, refracted or reflected, a small amount is scattered at the molecules. 

Many compounds exhibit near-IR and mid-IR absorption. By using IR transparent optical fibers, detection of an absorption band in the IR region is possible for optical sensing. Both direct sensing using the absorption property of the analyte or indicator sensing are widely exploited. Most mid-IR sensing schemes are based on the principles of internal reflection spectroscopy, or the attenuated total reflection (ATR) [3,14-21],... 

The book starts with a short introduction to the fundamentals of optical spectroscopy, (Chapter 1) describing the basic standard equipment needed to measure optical spectra and the main optical magnitudes (the absorption coefficient, transmittance, reflectance, and luminescence efficiency) that can be measured with this equipment. The next two chapters (Chapters 2 and 3) are devoted to the main characteristics and the basic working principles of the general instrumentation used in optical spectroscopy. These include the light sources (lamp and lasers) used to excite the crystals, as well as the instrumentation used to detect and analyze the reflected, transmitted, scattered, or emitted light. 

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