Reflectance spectroscopy types

Chapter 15, Differential Reflection Spectroscopy for Detection of Explosive Materials, describes a quite different type of technology that became available just as this book was in press. It is different from all the others described herein because it seeks to remotely locate and identify the explosive molecules in situ, whereas all the other trace sensing technologies require that some molecules be taken into the apparatus, ingested, in order to be sensed. This approach presents exciting possibilities, but is just emerging, with no field experience yet. 

The ozonide ion O3 has been clearly characterized by EPR and reflectance spectroscopy. Labeling experiments with 170 indicate that the O3 species contains three inequivalent oxygens forming a bond angle of about 110° and that it decomposes slowly at room temperature to form O ". A second type of species has been reported as O3 but has very different characteristics, since it is stable only at low temperatures and labeling experiments with 170, which indicate two equivalent nuclei, are difficult to interpret the balance of the evidence points toward a more complex polyoxygen species (see Section V,A). The data for O4 indicates that it is likely to exist on the surface under special conditions and we expect to see this confirmed by further studies. 

Internal reflection spectroscopy is widely applied for on-line process control. In this type of application, the chemical reactor is equipped with an internal reflection probe or an IRE. The goal of this type of application is the quantification of reactant and/or product concentrations to provide real-time information about the conversion within the reactor. In comparison with other analytical methods such as gas chromatography, high-pressure liquid chromatography, mass spectrometry, and NMR spectroscopy, ATR spectroscopy is considerably faster and does not require withdrawal of sample, which can be detrimental for monitoring of labile compounds and for some other applications. 

Different types of oxygen are distinguished also by Mitchell and Trifiro [219], who studied catalysts and model compounds by IR and UV reflectance spectroscopy. Activity appears to be correlated with Mo05 species which contain three terminal (multi-bonded) oxygen atoms ( Ot ). Moreover, a Mo—O bond was detected which is even weaker than Mo—O—Mo in Mo03 and assigned to Mo—O—Bi. Following the ideas of Schuit [281],... 

Based on steady-state and time-resolved emission studies, Scaiano and coworkers have concluded that silicalite (a pentasil zeolite) provides at least two types of sites for guest molecules [234-236], The triplet states of several arylalkyl ketones and diaryl ketones (benzophenone, xanthone, and benzil) have been used as probes. Phosphorescence from each molecule included in silicalite was observed. With the help of time-resolved diffuse reflectance spectroscopy, it has been possible to show that these triplet decays follow complex kinetics and extend over long periods of time. Experiments with benzophenone and arylalkyl ketones demonstrate that some sites are more easily accessed by the small quencher molecule oxygen. Also, diffuse reflectance studies in Na + -X showed that diphenylmethyl radicals in various sites decay over time periods differing by seven orders of magnitude (t varies between 20/is and 30 min) [237]. 

McFadden, L. A., Gaffey, M. J., Takeda, H., Jackowski, T. L. Reed, K. L. (1982) Reflectance spectroscopy of diogenite meteorite types from Antarctica and their relationship to asteroids. Mem. Nat. Inst. Planet. Res., 25,188-206. 

Because of the dependence of the PL intensity of TiC>2 on the nature of the gas-phase molecules introduced (alcohols) and its reversibility upon elimination of the molecules by flowing dinitrogen, there is hope that such an effect can be applied to gas sensors. With the combined use of several techniques (PL, time-resolved femtosecond diffuse reflectance spectroscopy, multiple internal reflection IR absorption), the dynamics and role of photogenerated electrons and holes in the absence or presence of metals (notably platinum) are now better understood, at both the gas-solid and liquid-solid interfaces. It is also likely that not only TiOz, but other types of semiconductors will be more thoroughly investigated in the future. 

Infrared spectra may be obtained for gases, liquids, orsolids. For transmittance infrared spectroscopy, the sampling techniques may involve a solution, a film, amull, or a pellet, depending on the type of sample. Reflectance spectroscopy differs from transmittance spectroscopy in that infrared radiation reflected from the surface of a material is studied. With a proper sampling accessory (obtainable from commercial sources), the materials analyzed by reflectance techniques normally require little or no sample preparation. The method is non-destructive, non-invasive, and very useful for analyzing materials that are too thick or have too much absorbance to be analyzed by transmittance spectroscopy. 

Different types of reflectance spectroscopy depend upon the reflecting behavior of the radiation on the solid. Fig. 8 illustrates various categories used to distinguish techniques for reflecting radiation off solids. Specular reflection spectroscopy is used to measure the reflectance spectrum of a smooth, glossy surface. In reflection-absorption spectroscopy, the radiation passes through a thin surface film on a reflective... 

The structures were grown in an ultra high vacuum (UHV) chamber VARIAN with a base pressure of 2-10 °Torr equipped with differential reflectance spectroscopy (DRS) [3] for a study of optical properties of the samples. Samples were cut from n-type 0.3 D cm Si(l 11) substrates. The silicon was cleaned by flashes at 1250 °C (7 times). Surface purity was controlled by AES. RDE was carried out at 500 °C, 550 °C, and 600 °C. The Cr deposition rate was about 0.04 nm/min controlled by a quartz sensor. An additional annealing during 2 min at 700 °C was done for all samples before the growth of silicon epitaxial cap layer. 

The optical spectroscopic techniques are feasible only for suitably transparent solids except where diffuse reflectance spectroscopy can be used. ESR requires nonconducting samples, so that metals can generally not be studied by either type of spectroscopy. 

For some sample types, for example a coated substrate, it is not possible to collect an infrared transmission spectrum, whereas in some cases (e.g. when there are concerns over the effects of sample preparation) it may be more desirable to collect a reflected spectrum. The most popular reflection techniques nowadays are internal reflection spectroscopy (IRS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)... 

The exact location in the micelle at which solubilization occurs (i.e., the locus of solubilization) varies with the nature of the material solubilized and is of importance in that it reflects the type of interaction occurring between surfactant and solubilizate. Data on sites of solubilization are obtained from studies on the solubilizate before and after solubilization, using X-ray diffraction (Hartley, 1949 Philipoff, 1950), ultraviolet spectroscopy (Reigelman, 1958), NMR spectrometry (Eriksson, 1963, 1966), and fluorescence spectroscopy (Saito, 1993 Paterson, 1999). Diffraction studies measure changes in micellar dimensions on solubilization, whereas UV, NMR and fluorescence spectra indicate changes in the environment of the solubilizate on solubilization. Based on these studies, solubilization is believed to occur at a number of different sites in the micelle (Figure 4-2) (1) on... 

Spectroscopic characterization of the zeolites. Diffuse reflectance spectroscopy reaches the spectral range comprised between 4000 and 40000 cm-1. Two types of transitions are of interest. In the first place, 4000-10000 cm-1 region contains vibrations associated with the OH groups combination bands near 4500-5000 cm-1 and the first overtone 2v0h near 72 00 cm-1. Secondly, the d-d transitions of the nickel ions show up between 4000 and 30000 cm-. ... 

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