 Photochemical quantum yield

The photochemical quantum yield measures the number of specified chemical events per excitation photon absorbed by the system. For an integrated product measurement,Counts $N$ or amounts of substance $n$ must be used consistently. State the product or reactant-change event, excitation wavelength and experimental conditions. A wavelength-specific yield strictly refers to monochromatic excitation.^[ref]

Absorbed and incident light are different denominators. Under the simple transmission model, the fraction absorbed at absorbance $A$ is $1-10^{-A}$. If $10\mu \mathrm{mol}$ of photons arrive at a sample with $A=1$, then $9\mu \mathrm{mol}$ are absorbed, neglecting reflection and scattering. Formation of $3\mu \mathrm{mol}$ of the chosen product gives $\Phi =1/3$; dividing by incident exposure instead gives $0.30$. These are invented bookkeeping values, not a measured reaction yield.^[ref]

A measurement therefore combines two calibrations: quantify the chemical change and determine absorbed photon exposure. A differential yield uses the reaction rate divided by absorbed photon flux; an integrated yield averages the specified change over an exposure. Changing absorption as products accumulate can make those quantities differ.

Photoisomerization supplies one application, but the definition requires the chosen event rather than an assumption that every absorbed photon makes one permanent product. A yield for chemical conversion also differs from a fluorescence quantum yield, which counts emitted photons. The numerical example does not impose a universal upper bound on every possible definition of a photochemical event.