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LinearAbsorption[sys, ] gives the formula for absorption of weak light of frequency by a system sys. 
LinearAbsorption[sys, , _{P}] finds absorption assuming Doppler broadening with width _{D}. 
LinearAbsorption[sys, , _{D}, _{P}] finds absorption assuming pressure broadening with width _{P} in addition to Doppler broadening. 
Absorption assuming a Doppler width _{D}. Here we write the light frequency in terms of the detuning .
