Blackbody radiation and quantum mechanics If I place a particle with more energy levels (of the order of kT) in a well defined state, in a thermal bath at temperature T, how will the blackbody radiation affect the internal state of the particle i.e. will the distribution be classical or QM? Basically, if I prepare that particle in that state, let it there a long enough time and measure it and I repeat this many times, I expect the populated energy levels to be given by a Boltzman distribution. In the end, did each particle have a well defined energy, but different particles had different energies. Or all the particles ended up in the same superposition of different energies with weights given by the Boltzman distribution, and when I measure them I make the energy collapse according to that weight? So, is the particle after that interaction with the thermal bath in a well defined state or in a superposition? Thank you!
 A: 
in a thermal bath at temperature T, how will the blackbody radiation affect the internal state of the particle i.e. will the distribution be classical or QM?

A thermal bath belongs to a thermodynamic mathematical model, and the mathematical model is emergent from the statistical distribution of a large number of particles. A particle with specific  energy levels obeys quantum mechanical equations.
This means that an atom with energy levels, lets make it simple, in a gas, can only interact quantum mechanically. In the case of the black body radiation you are discussing, it may scatter with a photon from the black body distribution of energy, and depending on the energy of the photon, the scattering may be elastic, inelastic, or change an energy level of the electron in the atom. This can happen independently to different atoms.

So, is the particle after that interaction with the thermal bath in a well defined state or in a superposition?

The only interaction with the thermal bath would be with a single  photon of the black body radiation, and superposition does not come into the problem.
