Basics of Laser Physics pp 85-96 | Cite as

# Basis of the Theory of the Laser: The Einstein Coefficients

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## Abstract

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).

## Keywords

Transition Frequency Thermal Equilibrium Spontaneous Emission Mode Density Blackbody Radiation
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