Species concentration, spontaneous Raman

The inelastic processes - spontaneous Raman scattering (usually simply called Raman scattering), nonlinear Raman processes, and fluorescence - permit determination of species densities as well as temperature, and also allow one, in principle, to determine the temperature for particular species whether or not in thermal equilibrium. In Table II, we categorize these inelastic processes by the type of the information that they yield, and indicate the types of combustion sources that can be probed as well as an estimate of the status of the method. The work that we concentrate upon here is that indicated in these first two categories, viz., temperature and major species densities determined from vibrational Raman scattering data. The other methods - fluorescence and nonlinear processes such as coherent anti-Stokes Raman spectroscopy - are discussed in detail elsewhere (5). 

Kreutner et al., 1983, 1987 Stephenson and Blint, 1979). A review of recent progress in this field has been provided by Eckbreth (1988) and Laurendeau (1988). Spontaneous Raman scattering lends itself to the measurement of concentrations of species above a few percent and to the determination of temperature. It can therefore also be used to investigate mixing processes (Leipertz, 1981b, 1981c). 

Just like IR spectroscopy, Raman can detect small. X-ray amorphous zeolite particles. Therefore RWan has been used to examine both the liquid and the solid phase of zeolite synthesis mixtures [28], Ex situ methods (with separation of solid and liquid) and in situ methods have been applied. In studying the liquid phase [10-11], one should remember that (i) minimum concentrations for detection of spontaneous Raman from liquids are typic ly 0.05 - 0.1 M, [15-27, 29] (ii) that the cross-section of the Al(OH)4 species is much stronger than e.g. for silicate or aluminosilicate anions [30]. Thus species which are present in low concentration or with variable structures may easily be overlooked in Raman spectra of the synthesis liquors. 

Laser-based spectroscopic probes promise a wealth of detailed data--concentrations and temperatures of specific individual molecules under high spatial resolution--necessary to understand the chemistry of combustion. Of the probe techniques, the methods of spontaneous and coherent Raman scattering for major species, and laser-induced fluorescence for minor species, form attractive complements. Computational developments now permit realistic and detailed simulation models of combustion systems advances in combustion will result from a combination of these laser probes and computer models. Finally, the close coupling between current research in other areas of physical chemistry and the development of laser diagnostics is illustrated by recent LIF experiments on OH in flames. 

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