where K (also denoted ) is measured in cm-1, is the extinction cross section, n is the number density, and is the opacity. The extinction coefficient at standard temperature and pressure (in cm-1) is
where is the density of the absorbing gas in g cm-2, is the density of the absorbing gas at standard temperature and pressure in g cm-2, n is the molecular number density in cm-3, is the molecular number density at standard temperature and pressure (the Loschmidt number) in cm-3, and m is the molecular mass in g.
The molecular extinction coefficient in cm-2 (also called extinction cross section) is
The mass extinction coefficient in cm2 g is
The volume extinction coefficient in cm-1 is
Corrected for stimulated emission, the specific extinction coefficient (in m-1) is given by
where and are Einstein coefficients, h is Planck's constant, is the frequency, and is the lineshape in Hz-1. Re-expressing the Einstein coefficients, becomes
Absorbance, Einstein Coefficients, Emissivity, Extinction, Extinction Coefficient, Loschmidt Number, Opacity, Planck Law
