The photostationary state of a reversible photochemical reaction is the equilibrium chemical composition under a specific kind of electromagnetic irradiation (usually a single wavelength of visible or UV radiation).
[1] It is a property of particular importance in photochromic compounds, often used as a measure of their practical efficiency and usually quoted as a ratio or percentage.
The position of the photostationary state is primarily a function of the irradiation parameters, the absorbance spectra of the chemical species, and the quantum yields of the reactions.
The photostationary state can be very different from the composition of a mixture at thermodynamic equilibrium.
For instance, although cis-stilbene is "uphill" from trans-stilbene in a thermodynamic sense, irradiation of trans-stilbene results in a mixture that is predominantly the cis isomer.
[5] The rate of a photochemical reaction is proportional to the absorption cross section of the reactant with respect to the excitation source (σ), the quantum yield of reaction (Φ), and the intensity of the irradiation.
The ratio of the rates of the forward and backwards reactions determines where the equilibrium lies, and thus the photostationary state is found at:
If (as is always the case to some extent) the compounds A and B have different absorption spectra, then there may exist wavelengths of light where σa is high and σb is low.
Likewise, wavelengths that give photostationary states of predominantly A may exist.