How is that if the orbital electron was in its ground state, and absorbs a photon raising to a higher energy level, before reemitting the photon. does it not return to its ground state. The total energy of the electron and photon before the absorption and after its emission must the the same. If not and a photon with the same energy is emitted where would the extra energy come fromYep. The only thing I can think of is orbital electron photoabsorption and reemission, which is fully reversible, but if you do that, the behaviour of the state you're probing is now the excited state, which is definitely not absolute zero, even if the initial and final temperatures were both at absolute zero.