A photolysis reaction involves the breaking of a chemical bond in a molecule by an incident photon (Pilling and Seakin, 1995, Bozó, 2009, Mészáros, 1997). The reaction is written

(R3.6) 
and the rate of this reaction is calculated as
, 
(3.10) 
where k is the photolysis rate coefficient.
Consider an elemental slab of air of vertical thickness dz and unit horizontal area. The slab contains [X]dz molecules of X (where [X] denotes the number density). A photon incident on a molecule of X has a probability of being absorbed, where A is the crosssectional area of the molecule and is the absorption crosssection (units of cm^{2} molecule^{1}) which defines the absorption characteristics of X. The molecules of X in the elemental slab absorb a fraction of the incoming photons. We define the actinic flux I as the number of photons crossing the unit horizontal area per unit time from any direction (photons cm^{−2} s^{−1}) and the quantum yield q_{X} (units of molecules photon) as the probability that absorption of a photon will cause photolysis of the molecule X. The number of molecules of X photolyzed per unit time in the slab is . To obtain the photolysis rate constant k, we divide by the number [X]dz of molecules of X in the slab:
. 
(3.11) 
Absorption crosssections and quantum yields vary with wavelength. For polychromatic radiation, as in the atmosphere, equation (3.11) must be integrated over the wavelength spectrum:
. 
(3.12) 
where is the actinic flux distribution function.