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The primary quantum yield

المؤلف:  Peter Atkins، Julio de Paula

المصدر:  ATKINS PHYSICAL CHEMISTRY

الجزء والصفحة:  ص847-848

2025-12-27

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The primary quantum yield

 We shall see that the rates of deactivation of the excited state by radiative, non-radiative, and chemical processes determine the yield of product in a photochemical reaction. The primary quantum yield, φ, is defined as the number of photophysical or photo chemical events that lead to primary products divided by the number of photons absorbed by the molecule in the same interval. It follows that the primary quantum yield is also the rate of radiation-induced primary events divided by the rate of pho ton absorption. Because the rate of photon absorption is equal to the intensity of light absorbed by the molecule (Section 13.2), we write

φ=

A molecule in an excited state must either decay to the ground state or form a photo chemical product. Therefore, the total number of molecules deactivated by radiative processes, non-radiative processes, and photochemical reactions must be equal to the number of excited species produced by absorption of light. We conclude that the sum of primary quantum yields φ_i for all photophysical and photochemical events i must be equal to 1, regardless of the number of reactions involving the excited state. It follows that

∑ _i φ_i = ∑

It follows that for an excited singlet state that decays to the ground state only via the photophysical processes described in Section 23.7(a), we write

φ_f +φ_IC +φ_ISC +φ_p=1

where φ_f, φ_IC , φ_ISC, and φp are the quantum yields of fluorescence, internal conversion, intersystem crossing, and phosphorescence, respectively. The quantum yield of pho ton emission by fluorescence and phosphorescence is φ_emission = φf + φp, which is less than 1. If the excited singlet state also participates in a primary photochemical reaction with quantum yield φ_R, we write

φ_f + φ_IC +φ_ISC +φ_p+φ_R=1

We can now strengthen the link between reaction rates and primary quantum yield already established by eqns 23.28 and 23.29. By taking the constant I_abs out of the sum mation in eqn 23.29 and rearranging, we obtain I_abs =∑_ivi. Substituting this result into eqn 23.29 gives the general result

φi =

Therefore, the primary quantum yield may be determined directly from the experimental rates of all photophysical and photochemical processes that deactivate the excited state.