Dissipation of excess energy

In this chapter, the discussion will center on the reactions of excited states, rather than on the other routes available for dissipation of excess energy. The chemical reactions of photoexcited molecules are of interest primarily for three reasons ... 

Cadoret, J. C., R. Demouliere, J. Lavaud et al. (2004). Dissipation of excess energy triggered by blue light in cyanobacteria with CP43 (IsiA). Biochim Biophys Acta 1659(1) 100-104. 

Grinding of Materials. The mechanical disintegration of a soUd leads to the formation of a new surface. The formation of this surface begins with the rupture of interatomic bonds and is concluded with an ordering of structure of the layer near the surface. As a rule, the formation of the new surface is accompanied by dissipation of excess energy as evidenced by the evolution of heat, the emission of electrons, ions, or neutral particles, luminescence, and other phenomena. 

Figure 5.6 Fluorescence. Absorption of incident radiation from an external source causes excitation of the analyte to state 1 or 2. Excited species can dissipate the excess energy by emission of a photon or by radiationless processes (dashed lines). The frequencies emitted correspond to the energy differences between levels...

Chloromethan and ethane, formed in the terminating steps, can dissipate their excess energy through vibrations of their C-H bonds. 

The simple diatomic chlorine that is formed must dissipate its excess energy rapidly by colliding with some other molecule or the walls of the container. Otherwise it simply flies apart again. 

Most of the energy associated with an incident x-ray or y-ray is absorbed by ejected electrons. These secondary electrons are ejected with sufficient energy to cause further ionizations or excitations. The consequences of excitations may not represent permanent change, as the molecule may just return to the ground state by emission or may dissipate the excess energy by radiationless decay. In the gas phase, excitations often lead to molecular dissociations. In condensed matter, new relaxation pathways combined with the cage effect greatly curtail permanent dissociation. Specifically in DNA, it is known that the quantum yields for fluorescence are very small and relaxation is very fast [6]. For these reasons, the present emphasis will be on the effects of ionizations. 

Consequently, the formation of a labile addition product, Sensrad. .. 02, was postulated. This should transfer its oxygen to a substrate A via a collision complex Sensrad... 02... A, which collapses into the ground-state sensitizer and an energy-rich AOa. The latter stabilizes to A02 by dissipating its excess energy as heat.26 Since photooxygenation reactions were shown to occur even at very low temperatures with relatively high quantum yields, the identity of Sensrad with a sensitizer in the excited triplet state was assumed.61,68-70... 

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