Atomic transitions and electromagnetic waves The interaction of an assembly of identical atoms with a radiation field spontaneous emission stimulated emission absorption

Transition rates the photon energy density per unit frequency The interaction of an assembly of identical atoms with a radiation field transition rates the population densities of levels 1 and 2 spontaneous emission stimulated emission absorption

Determination of the Einstein coefficients We consider the case where the atoms are in a thermal equilibrium with a blackbody radiation field (Planck’s law) (total downward transition rate = total upward transition rate) (Boltzmann statistics)

Conclusions for the LASER We need a large photon concentration, this is achieved in an optical cavity We need population inversion, i.e., The LASER principle is based on non-thermal equilibrium.