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Excerpt from Field Guide to Spectroscopy
There are two spectroscopically important processes: absorption and emission.
Absorption is when an atomic or molecular process takes in a photon and goes to a higher energy such that the increase in energy equals the energy of the photon:
In 1917, Albert Einstein defined a rate of stimulated absorption as
where ρ(ν) is the density of radiation having frequency ν, c is the concentration of the absorbers in the lower energy level, and B is the Einstein coefficient of stimulated absorption and is given by
Absorption of photons is probably the most common measurement in spectroscopy. The absorptivity of a chemical substance, ε(λ), is a measure of how strongly that substance absorbs light of a particular wavelength. It is commonly expressed in molar terms and is also referred to as the molar absorptivity or the molar extinction coefficient. The absorbance, A(λ), of a sample depends on the molar absorptivity, the concentration of the chemical substance, and the distance the light passes through the sample.