High-density two-level radiative cooling
= High-density two-level radiative cooling
When collisions thermalize an optically thin two-level atom, the upper fraction is $f_u=g e^{-\theta/T}/(1+g e^{-\theta/T})$ and the emitted power per atom is $A_{21}\Delta E f_u$. Its logarithmic <temperature> slope is $(\theta/T)(1-f_u)>0$. <Radiative cooling> still occurs, but collision frequency no longer sets the emitted power as in the low-<density> limit.