Einstein Absorption and Emission

Consider an atom capable of absorption and emission. Einstein defined the following  

You will note that he specified both excited and ground states in teims of both numbers and energies of the atoms involved. 

Einstein then proceeded to define an Intensity of absorption as  

Bnm = Einstein coefficient of Absorption Bmn = Einstein coefficient of Induced Emission Amn = Einstein coefficient of Spontaneous Emission (from one pole to the other in the order given) 

n is the ground state here while m is the upper energy state. Einstein noted two types of emission and had to define two (2) types of emission coefficients. These were delineated as two types of emission processes  

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