Photochemical reactions chemical change rate

Mechanisms of photochemical decompositions of disulfides are still uncertain in many cases, due to the lack of quantitative data on rates of product formation and to conflicting reports on the nature of the reaction products. An obvious complicating factor in this system is the difficulty of assessing the importance of recombination reactions which leads to no net chemical change, i.e. 

In many cases a chemical compound may, depending on the conditions of the photochemical reaction, behave either as a photo-initiator or as a photosensitizer. Benzophenone is a typical example. For this reason some authors do not distinguish between these two classes of compounds, assuming that their positive influence on the changes of the rate of photochemical reactions represents the sensitization of these reactions. 

Photocatalysis can be defined as follows A change in the rate of chemical reactions or their generation imder the action of light in the presence of substances called photocatalysts - that absorb light quanta and are involved in the chemical transformations of the reactants [4]. Typical photocatalysts or photosensitisers are semiconductor materials. There are many chemical compounds which can act as photocatalysts, but only a very few of these materials are photochemically and chemically stable semiconductor photocatalysts, one compound dominates titania (titanium dioxide) Ti02. 

The rate of chemical reaction (thermal or photochemical) is, the velocity by which reaction proceeds. All the photochemical reactions go through excited intermediate states. The life-time of reactive excited states effects the rate of a reaction directly because this is the time which is provided for the chemical transformation. Mostly photoexcited species undergo chemical change but there are equal chances of photophysical deactivation also. Hence, there is difference in the number of molecules get excited and molecules converted into product. Thus, it is very important to make a relation between rate constant of photochemical reaction and life-time of reactive energy state of reactant. 

The problems in analyzing the results of photochemical (and chemical) activation experiments on unimolecular reactions (see Section 1.4) are closely related to those which arise in studies of fluorescence quenching. Excitation of a polyatomic molecule with monochromatic radiation can produce species with a well-defined energy (e) which exceeds the critical energy (e°) for some unimolecular chemical change. The usual goal in such experiments is to find the specific rate constant, k e defined by... 

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