  The temporal evolution of the levels in a three-level laser is
macroscopically described by a closed set of rate equations. Here, we
complement this picture by providing a model which describes through chemical
reactions how the levels are evolving. In most textbooks, the chemical
potential is introduced as a concept in quantum statistics. In this paper, we
alternatively base its definition on population densities of the excited states
in a dopant atom. Then the chemical reaction model delivers a clear and
intuitive framework to define the further thermodynamic characteristics of the
atom-field interactions such as the chemical potential of the photon and the
photon entropy.
