We demonstrated that controlled periodic illumination (CPI) is unable to increase the quantum yield (or photoefficiency) of the photocatalytic process. Nevertheless, choosing the appropriate experimental conditions and the use of CPI coupled with a simplified kinetic model allowed us to extract the lifetime of the species involved in substrate oxidation. These figures are usually elusive, even though there is a wide collection of hypotheses, guesses, and speculations in the literature. From our analysis, we defined a surface photocatalytic activity index. The latter consists of the ratio between the surface-defined kinetic constant for substrate oxidation and surface recombination and is independent of other experimental conditions, such as irradiance and catalyst concentration.

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