Einstein Coefficients

coefficients that characterize the probabilities of radiative quantum transitions. The Einstein coefficients were introduced by Albert Einstein in 1916 in the course of his formulation of the theory of the emission and absorption of radiation by atoms and molecules on the basis of the concept of photons. Einstein was the first to advance the idea that stimulated emission existed. The probabilities of spontaneous emission, absorption, and stimulated emission are characterized by the coefficients A_ti, B_ik, and B_ki, respectively, in which the subscripts indicate the direction of the transition between the upper energy level E_k and the lower level E_i. The relations between the Einstein coefficients were originally obtained by Einstein in the course of the derivation of Planck’s radiation law from the thermodynamic equilibrium between matter and radiation.