Ernest Bauer & Ta-You Wu 
Cooling of a Gas by Radiation [PDF ebook] 

Whilst the greatest effort has been made to ensure the quality of this text, due to the historical nature of this content, in some rare cases there may be minor issues with legibility. To simplify the calculations we shall consider a gas composed of atoms having two electronic states 1 and 2 and distributed in space in a given manner (such as the gas in the atmosphere). Transitions between these two states can take place by collisions and by emission or absorption of radiation. In a.