Why does photolysis require a powerful lamp

It is advisable to employ a high-power lamp when performing a photochemical reaction because it produces more photons than a low-power lamp. Its flux is greater. When we looked at the laws of photochemistry, we saw how the second law stated the idea that when a species absorbs radiation, one particle is excited for each quantum of radiation absorbed. This (hopefully) obvious truth now needs to be investigated further. 

The quantum yield t is a useful concept for quantifying the number of molecules of reactant consumed per photon of light. It may be defined mathematically by 

Its value can lie anywhere in the range 10-6 to 106. A value of 10-6 implies that the photon absorption process is very inefficient, with only one molecule absorbed per million photons. In other words, the energetic requirements for reaction are not being met. 

Conversely, a quantum yield P of greater than unity cannot be achieved during a straightforward photochemical reaction, since the second law of photochemistry clearly says that one photon is consumed per species excited. In fact, values of t 1 indicate that a secondary reaction(s) has occurred. A value of t 2 implies that the product of the photochemical reaction is consumed by another molecule of reactant, e.g. during a chain reaction, with one photon generating a simple molecule of, say, excited chlorine, which cleaves in the excited state to generate two radicals. Each radical then reacts in propagation reactions until the reaction mixture is exhausted of reactant. 

To help clarify the situation, we generally define two types of quantum yield primary and secondary. The magnitude of the primary quantum yield refers solely to the photochemical formation of a product so, from the second law of Photochemistry, the value of 0(primary) cannot be greater than unity. 

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