Characterization of the Quasi-Stationary State of an Impurity Driven by   Monochromatic Light II - Microscopic Foundations
  From quantum mechanical first principles only, we rigorously study the time-evolution of a $N$-level atom (impurity) interacting with an external monochromatic light source within an infinite system of free electrons at thermal equilibrium (reservoir). In particular, we establish the relation between the full dynamics of the compound system and the effective dynamics for the $N$-level atom, which is studied in detail in [Bru-de Siqueira Pedra-Westrich, Annales Henri Poincar\'e, 13(6):1305-1370, 2012]. Together with [Bru-de Siqueira Pedra-Westrich, Annales Henri Poincar\'e, 13(6):1305-1370, 2012] the present paper yields a purely microscopic theory of optical pumping in laser physics. The model we consider is general enough to describe gauge invariant atom-reservoir interactions. 
J.-B. Bru and W. de Siqueira Pedra
math-ph math.MP quant-ph