Question

Einstein coefficients for inhomogeneously broadened laser?

Answer 1

Albert Einstein proposed that there are three processes occurring in the formation of an atomic spectral line. The three processes are referred to as spontaneous emission, stimulated emission, and absorption. With each is associated an Einstein coefficientwhich is a measure of the probability of that particular process occurring. 
                              Of these three only one is independent.  The other two depend upon the coefficient taken to be independent, plus the frequency of the transition and the statistical weights of the two associated terms.



                              
                             A21 [s-1] = "coefficient of spontaneous emission" = the probability per unit time that an atom in energy state 2 will spontaneously emit a photon and undergo a transition to energy state 1.

                             B12 [s-1 erg-1 cm2 sterad ] = "coefficient of absorption" = the probability per unit time per unit mean intensity at frequency ν0 that an atom in energy state 1 will absorb a photon of frequency ν0 and jump to energy state 2.

                            B21 [s-1 erg-1 cm2 sterad ] = "coefficient of induced emissions" = the probability per unit time per unit mean intensity at frequency ν0 that an atom in energy state 2 will emit a photon of frequency ν0 and jump to energy state 1.