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Photon with energy $E_{n_u}-E_n$ is absorbed by the atom in the state of n. At the same time the atom is excited from the state n to n$_u$. The number of this transition is proportional to the number of atoms in the state n, $N_n$, the number of photons passing and a cross-section of this absorption. This is written as
B_{n\rightarrow n_u} J_{\nu_{n\rightarrow n_u}} N_n,
\end{displaymath} (2.135)

where $J$ represents the average specific intensity of radiation with corresponding wavelength. This coefficient is called Einstein's $B$ coefficient for absorption.

Kohji Tomisaka 2007-07-08