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:
ΔE ≡ Ef - Ei = hν
In 1917, Albert Einstein defined a rate of stimulated absorption as
rate of stimulated absorption = B · ρ(ν) · c
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
where:
e—charge on electron, 1.6022×10-19C
ε0—permittivity of free space, 8.8542×10-12C2/J·m
h—Planck's constant, 6.6261×10-34J·s
me—mass of the electron, 9.1094×10-31 kg
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.
