Charge neutralization Chemiluminescence

Excited states may be formed by (1) light absorption (photolysis) (2) direct excitation by the impact of charged particles (3) ion neutralization (4) dissociation from ionized or superexcited states and (5) energy transfer. Some of these have been alluded to in Sect. 3.2. Other mechanisms include thermal processes (flames) and chemical reaction (chemiluminescence). It is instructive to consider some of the processes generating excited states and their inverses. Figure 4.3 illustrates this following Brocklehurst (1970) luminescence (l— 2)... 

From a practical standpoint, much of the interest in the role of excited states in ionic interactions stems from their importance in ionospheric chemistry.Ih In addition, it has been realized more recently that certain ion-neutral interactions offer a comparatively easy means of populating electronically excited reaction products, which can produce chemiluminescence in the visible or UV region of the spectrum. Such systems are potential candidates for practical laser devices. Several charge-transfer processes have already been utilized in such devices, notably He+(I,He)I + and He2+(N2,2He)N2+.3 Interest in this field has stimulated new emphasis on fundamental studies of luminescence from ion-neutral interactions. 

If so, one may expect products to result from chemical bond formation between the cation-radical-anion-radical pair, which are both paramagnetic and of opposite charge. In the latter route, there is a precedent for the formation of dioxetane intermediates of stable olefin cation radicals [51], as in the characterization by Nelsen and coworkers of a dioxetane cation radical from adamantylidene cation radical [52]. If a dioxetane is formed, either in neutral form or as a cation radical, the Ti02 surface can function in an additional role, that is, as a Lewis acid catalyst, to induce decomposition of the dioxetane. Since no chemiluminescence could be observed in these reactions, apparently Lewis acid catalysis provides a nonradiative route for cleavage of this high-energy intermediate. That Ti02 can indeed function in this way can be demonstrated by independent synthesis of the dioxetane derived from 1,1-diphenylethylene, which does indeed decompose to benzophenone when it is stirred in the dark on titanium dioxide. 

Finally, in analytical applications the luminescence is often used only to detect specific components of interest. The microemulsion is then employed to extract these components, e.g., polycyclic aromatic hydrocarbons [81], or to separate neutral metal ion complexes by electrophoresis with charged microemulsion droplets [82]. But it must not always be luminescence induced by external light chemiluminescence in microemulsions was also reported [83] to give an appreciable increase in the intensity compared to homogeneous solutions. 

While no mass analysis has yet been performed to distinguish between these paths, all of them demonstrate directly the occurrence of electron jumps to form charged intermediates. These paths can be viewed as events during the course of which the ions somehow missed the chance of recombining into neutral products. Also, if a recombination does occur, it will not always lead to the electronic ground state of the products. Thus formation of electronically excited alkali atoms or halide molecules can be expected and has, in fact, been observed [1] ("chemiluminescence ). 

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