The quantum yield of a photochemical reaction is defined as the ratio of the number of molecules that undergo the desired reaction to the number of photons absorbed by the system. In this case, 50% of the absorbed photons lead to the formation of the product. Therefore, the quantum yield of this reaction is 0.5 or 50%.Quantum yield and quantum efficiency are related but distinct concepts. Quantum yield, as mentioned earlier, is the ratio of the number of molecules that undergo the desired reaction to the number of photons absorbed. On the other hand, quantum efficiency is the ratio of the number of electrons generated by the photochemical reaction to the number of photons absorbed. In some cases, quantum yield and quantum efficiency can be equal, but they can also differ depending on the specific reaction and the processes involved.To experimentally determine the quantum yield of a photochemical reaction, you need to measure two things: the number of photons absorbed by the system and the number of molecules that undergo the desired reaction. The number of photons absorbed can be determined by measuring the decrease in light intensity after passing through the sample, while the number of molecules that undergo the desired reaction can be determined by monitoring the concentration of the product or the decrease in the concentration of the reactant.Once you have these two values, you can calculate the quantum yield by dividing the number of molecules that undergo the desired reaction by the number of photons absorbed. In this case, since 50% of the absorbed photons lead to the formation of the product, the quantum yield is 0.5 or 50%.