Basis of the Theory of the Laser: The Einstein Coefficients
According to Bohr’s atomic model (1911), which is based on spectroscopic investigations, transitions between discrete energy levels of an atom can lead to emission or absorption of radiation of a frequency that fulfills Bohr’s energy-frequency relation. In an absorption process, a photon is absorbed. In an emission process, a photon is emitted. Einstein found that the emission of a photon is possible by two different processes, spontaneous and stimulated emission, and that the coefficients describing the three processes — absorption, stimulated and spontaneous emission — are related to each other (Einstein relations).