Einsteins quantitative law

In a primary photochemical act there are as many broken down molecules as there are absorbed photons, which mean that the primary act can be written as  

Warning this law is only tme for the reaction in the direction from left to right If the reverse reaction is significant or the transmission factor is low there will not be as many molecules of AB broken down as there will be photons absorbed. It is said that the quantum yield is less than one because it is the ratio of broken down B molecules over the raunber of photons absorbed. 

The fact that the quantum yield is smaller or larger than one provides important irrformation on the mechanism. We have already seen factors that diminish the quantum yield. Other factors promote its increase. Reactions of high quantum yield are always exothermic reactions and are responsible for the discovery of chain reactions. Indeed, if the photochemicals act is the initiation step in the chain, as is most often the case, the amount of product that has reacted following the absorption of one photon can be considerable. These reactions can achieve quantum efficiencies 

The influence of temperature on photochemical reaction rates can be very different depending on the type of reaction, almost always following the same order of magnitude as the quantum yield. The primary act is never affected by temperature and thus the change of reaction rate with temperature is always due to other, thermal, steps of the mechanism. 

Consider first a reaction with simple, limited sequences. If the rate-determining step is the primaiy photochemical act, usually the first step, and conditions are far from equilibrium, we will have a reaction with a quantum yield with units and a reaction rate that is not influenced by temperature. 

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