Absorption Cross Section and Einstein Coefficients

A measure of the absorption strength is the absorption cross section Oik. Each photon passing through the circular area oik = nrfj around the atom is absorbed on the transition /) k). 

At thermal equilibrium the population follows a Boltzmann distribnlion. Inserting (2.18) yields the power absorbed within the volume AF = Az by a sample with molecular density N and temperature T out of an incident beam with the cross section A and intensity I 

In case of a collimated laser beam passing through the absorbing sample the volume element AF = A Az is the product of the laser beam cross section A and the absorption path length Az. 

however, possible to pump molecules into higher energy states by various excitation mechanisms such as optical pumping or electron excitation. This allows the measurement of absorption spectra for transition from these states to even higher molecular levels (Vol. 2, Sect. 5.3). 

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Relation Between Absorption Cross Section and Einstein Coefficients... 

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