Rate Constants and Their Relation with the Life-Time of Reactive Excited States

RATE CONSTANTS AND THEIR RELATION WITH THE LIFE-TIME OF REACTIVE EXCITED STATES  

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 photochemical reaction may be unimolecular such as photo dissociation and photo isomerisation, or bimolecular or polymolecular. The polymolecular photoreactions occurs between one photoexcited molecule with other molecules. 

The photochemistry is the study of chemistry of excited states. There are two types of excited states, singlet excited state and triplet excited state, having life-time very short about 10 S to 10 S and 10 to IS respectively. The efficiency of a photochemical reaction is expressed in terms of quantum efficiency ( )) of the reaction. Quantum efficiency or quantum yield is the ratio of rate of reaction and rate of absorption of radiation. 

Moles of product formed or moles of reactants consumed S cm Einstein of radiation absorbed S cm  

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