Secondary Processes Involving Excited Molecules

In general, the secondary processes involving excited molecules include light emission by an excited molecule (fluorescence), energy loss in collision with another molecule (fluorescence quenching) and interaction of an excited molecule with other molecules. 

Denoting the concentration of excited molecules by n and the rates of secondary processes by Vi 

If there is no predissociation or if it is unimportant, the stationary reaction rate is expressed as 

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Kuvshinskij et al. (1991) measured photogeneration efficiencies of poly(N-epoxypropylcarbazole) sensitized with rhodamine 6G. Kuvshinskij et al. argued that the photogeneration occurs via two concurrent mechanisms. The principal mechanism is a thermalization process that creates a bound electron-hole pair where the hole resides on one of the carbazole pendent groups and the electron on an excited dye molecule. A secondary process involves exciplex dissociation. The dissociation energy of the exciplex was reported as 0.54 eV. 

The basic processes which are responsible for reaction on irradiation of these covalent inorganic systems involve excitation and ionization of the molecules by the secondary electrons generated in the sample by Compton scattering. The primary species produced are excited molecule ions... 

Secondary photochemical processes are chemical reactions involving excited species (in either the triplet or singlet state) produced by electronic excitation of the molecule ... 

A particularly large amount of research has been devoted to mercury-sensitized photochemical reactions of hydrogen compounds (H2, hydrocarbons, etc), especially to decomposition and oxidation reactions. The secondary process of interaction between an excited mercury atom and a reactant molecule may be involved in these reactions by proceeding two paths Hg + HH Hg -f- R + H and Hg + RH -> HgH + R. 

The efficiency of any photophysical or photochemical process is a function of both the properties of the reaction environment and the character of the excited state species. The fundamental quantity which is used to describe the efficiency of any photo process is the quantum yield (0) it is useful in both quantifying the process and in elucidating the reaction mechanism. Quantum yield has the general definition of the number of events occurring divided by the number of photons absorbed. Therefore, for a chemical process 0 is defined as the number of moles of reactant consumed or product formed divided by the number of einsteins (an einstein is equal to 6.02 X 10 photons) absorbed. Since the absorption of light by a molecule is a one-quantum process, then the sum of the quantum yields for all primary processes occurring must be one. Where secondary reactions are involved, however, the overall quantum yield can exceed unity and for chain reactions reach values in the thousands. When values of 0 are known or can be measured for a specific photochemical reaction the rate can be determined from ... 

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