The effective area per target for absorbing incident radiation at a specified wavelength. For independent absorbers with number column , the absorption optical depth is . Absorption does not guarantee a particular chemical product; a channel's quantum yield is also needed in its photolysis rate coefficient.
For a closed two-level system with total population , excited-state lifetime and statistical sublevel populations, let . The steady rate equation gives absorption coefficient . For a uniform slab of thickness and transmission , integrating proves . Resolved sublevel optical pumping requires a more detailed model.
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The absorption cross section is a measure of the likelihood of a particle (such as a photon) being absorbed by a target, which can be an atom, molecule, or any medium. It quantifies the effective area that a particular absorber presents to incoming radiation, correlating the physical properties of the absorber with its ability to absorb electromagnetic radiation.