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Absorption
Photon with energy is absorbed by the atom in the
state of n.
At the same time the atom is excited from the state n to n.
The number of this transition is proportional to the number of
atoms in the state n, , the number of photons passing and
a crosssection of this absorption.
This is written as

(2.135) 
where represents the average specific intensity of radiation with
corresponding wavelength.
This coefficient is called Einstein's coefficient for absorption.
Kohji Tomisaka
20070708