Optical techniques basic concepts

We present the basic concepts and methods for the measurement of infrared and Raman vibrational optical activity (VOA). These two forms of VOA are referred to as infrared vibrational circular dichroism (VCD) and Raman optical activity (ROA), respectively The principal aim of the article is to provide detailed descriptions of the instrumentation and measurement methods associated with VCD and ROA in general, and Fourier transform VCD and multichannel CCD ROA, in particular. Although VCD and ROA are closely related spectroscopic techniques, the instrumentation and measurement techniques differ markedly. These two forms of VOA will be compared and the reasons behinds their differences, now and in the future, will be explored. 

Since the introduction of the STM a number of variations have been devised, such as ATM (atomic force microscope). The basic concept is that piezoelectric actuators move a miniature cantilever arm (with a nm-sized tip) across the sample while a non-contact optical system measures the deflection of the cantilever caused by atomic scale features. The deflection is proportional to the normal force exerted by the sample on the probe tip and images are generated by raster scanning the sample [201]. One application of this technique was to measure the thickness and size distribution of sub-micron clay particles with diameters in the 0.1 to 1 pm size range and thickness from 0,01 to 0.12 pm [202]. 

Aerosol science plays a key role in many different fields including (a) atmospheric sciences and air pollution, (b) Industrial production of pigments, fillers, and specially metal powders, (c) fabrication of optical fibers, (d) industrial hygiene, and (e) contamination control in the microelectronics and pharmaceuticals industries. Aerosols present in such applications can usually he considered as desirable or undesirable, but the same basic concepts apply to both types. Specialist. in the various applied fields increasingly make use of similar theoretical concepts and experimental techniques in solving aerosol problems. These common approaches are the focus of this book. 

Chapter 1 of the present volume provides the basic concepts related to the properties and characterization of the centres known as shallow dopants, the paradigm of the H-like centres. This is followed by a short history of semiconductors, which is intimately connected with these centres, and by a section outlining their electrical and spectroscopic activities. Because of the diversity in the notations, I have included in this chapter a short section on the different notations used to denote the centres and their optical transitions. An overview of the origin of the presence of H-related centres in crystals and guidelines on their structural properties is given in Chap. 2. To define the conditions under which the spectroscopic properties of impurities can be studied, Chap. 3 presents a summary of the bulk optical properties of semiconductors crystals. Chapter 4 describes the spectroscopic techniques and methods used to study the optical absorption of impurity and defect centres and the methods used to produce controlled perturbations of this absorption, which provide information on the structure of the impurity centres, and eventually on some properties of the host crystal. Chapter 5 is a presentation of the effective-mass theory of impurity centres, which is the basis for a quantitative interpretation... 

Fink JK (2005) Reactive polymers fundamentals and applications. William Andrew, New York Fitch R (1997) Polymer colloids. Academic Press, New York Freed KF (2005) Phase behaviour of polymer blends. Springer, Berlin Guo B, Ghalambor A (2005) Natural gas engineering handbook. Gulf, Houston, TX Hadjichristidis N, Pispas S, Floudas GA (2003) Block copolymers. Wiley, New York Hamley IW (2004) Development in block copolymer science and technology. Wiley, New York Hiemenz PC (1984) Polymer chemistry basic concepts. Marcel Dekker, New York Hou CT (2005) Handbook of industrial biocatalysis. CRC Press, Boca Raton, FL Hougham G (ed) (1999) Fluoropolymers. Kluwer Academic/Plenum Publishers, New York Jabbour GE et al (chairs/eds) (2001) Combinatorial and composition spread techniques in materials and device development II 22-23, 25 January 2001, San Jose, USA, spons. and publ. by SPIE, The International Society for Optical Engineering, SPIE Jutzi P, Schubert U (2003) Silicon chemistry - from the atom to extended systems. Weinheim, Wiley VCH... 

Figure 2 illustrates the basic concept of a typical pump-probe spectroscopy used in most ultrafast spectroscopy techniques. In its simplest form the output pulse train of an ultrafast laser is divided in two by a beam splitter. One pulse in train (called pump) first excites the sample under investigation. The second pulse train (called probe) will probe the sample with a suitable time delay with respect to the pump by introducing an optical delay in its path and some optical property (e.g., reflectivity, absorption, Raman scattering, luminescence, optical nonlinear responses) of the sample is then detected to investigate the changes produced by the pump. In most of the time-resolved pump-probe experiments, the time resolution is limited only by the pulse width of the laser or the jitter between the laser systems. 

During the early stages in the development of stereochemical concepts near the beginning of this century, chemists had to rely on rather indirect methods to deduce basic stereochemical information, such as the absolute configuration of a chirality center at carbon. In most cases, arguments were based on the chemical transformation of a molecule of interest to a molecule of known stereochemistry. Although optical rotation was a valuable tool in this process, absolute stereochemistry could not be deduced directly by spectroscopic techniques. The fine details of stereochemistry such as preferred conformational states and their rel-... 

Numerous measurement approaches use these basic six techniques. For instance, a binding event between the target agent and the test probe can be measured using the differences in mass between the individual molecules and the bound complex. Alternatively, an optical label can be attached to the probe and detection achieved after physical filtering of the molecular complex to provide separation from the labeled probe. Each of the six basic techniques has resulted in numerous instrument prototypes and concepts for detection and identification. 

The most widely used technique for the estimation of amino acid isomers is gas-liquid chromatography. Two basic strategies have been used to separate isomers by this technique. Both approaches appeared in the mid-1960 s, and both involve derivatiza-tion of the amino acids to suitably volatile acyl-esters (46,47). One method is similar in concept to the separation of disastereo-mers by ion exchange chromatography discussed above. In one step of the two step derivatization, the amino acids are esterified with an optically active agent. This procedure creates molecules with two centers of asymmetry which can be separated on a non-optically active liquid (stationary) phase (46). This method allows any laboratory equipped with a gas chromatograph to perform Isomer analyses. 

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