4. Energy level population

4.1 Boltzmann's principle

Quantum mechanics admits that absorption and emission mechanisms take place independently for each isolated atom. A photon is emitted by a single atom with an energy hν depending on the difference in the energy levels involved. However, it is not possible to isolate an atom or a photon experimentally, and all experiments involve not only a large number of photons that can be evaluated by their spectral energy density, but also a collection of atoms whose number per unit volume can be calculated. These atoms are placed in a "thermal bath" defined by the temperature of the medium, which supplies thermal energy to induce absorption transitions towards excited levels, leading to thermal equilibrium. We therefore need to know the laws of statistical mechanics...