Applications of Laser Spectroscopy
The experimental advantages of laser spectroscopy regarding spectral power density and spectral resolution have brought about a great variety of applications in many scientific and technical fields. The selection of examples presented in this chapter is b
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The experimental advantages of laser spectroscopy regarding spectral power density and spectral resolution have brought about a great variety of applications in many scientific and technical fields. The selection of examples presented in this chapter is by far not complete but intends to illustrate the impact of laser spectroscopy on the development of new experimental techniques in chemistry, biology, and environmental sciences. The importance of laser spectroscopic applications is emphasized by the publication of many monographs, review papers, or conference proceedings on this subject. For more detail ed i nformati on the reader i s therefore referred to the cited 1iterature [1.17,19; 14.1-6J.
14.1 Laser Photochemistry There are many ways in which lasers may be utilized in chemistry. Of particular interest, and probably of great economical importance in the near future, is the possibility of enhancing or catalyzing specific chemical reactions by selective excitation of the reactands via optical pumping with lasers. Another area of lasers in chemistry is the study of internal state distributions of reaction products using laser-induced fluorescence techniques (see Sect.8.7). The dependence of this distribution on the internal energy of the reactands or on their translational energy allows far-reaching conclusions about the reaction pathways and the potential surfaces of the intermediate state. A very interesting field comprises spectroscopy investigations of energy transfer processes (see Chap.12) which allow a more detailed insight into the nature of inelastic and reactive collisions [14.6aJ. One example is the study of excitation and deactivation processes in chemical lasers or in energy transfer lasers in the infrared and visible regions. Let us first consider laser-induced chemical reactions. The excitation energy of one or several reactands which initiates and drives the chemical W. Demtröder, Laser Spectroscopy © Springer-Verlag Berlin Heidelberg 1981
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A + B C ' - 8 - AB '+ C
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/' 1 pump laser
fluorescence
(0)
A+ BC (b)
+C fluorescence
Fig. 14.1a,b. Possible applications of lasers to chemical reactions. (a) Enhancement of reaction rate by laser excitation of a reactant before the collision, and subsequent monitoring of internal state distribution of the reaction product by LIF. (b) Laser-induced collisional energy transfer (LICET)
reaction is supplied by absorption of one or of several laser photons. For these processes the time elapsed between absorption of laser photons and completion of the desired reaction is of crucial importance. The energy pumped into a selected level of a reactant molecule can be dissipated in several ways before the desired reaction takes place. It can be lost by spontaneous emission, it may be redistributed among the numerous degrees of freedom of a large excited molecule, or it may be thermalized and shared with other molecults through collisional deactivation. The time scales of these processes depend on the kind of molecules, the amount of exc
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