Are photoionization from ground and recombination to ground consider a reversible processes in the thermodynamic sense?
Ignoring any quantum broadening effects, consider a closed system of one Hydrogen and one ionizing photon. The photon could ionize the atom, which would later recombine emitting the same energy photon that was absorbed. This seems to be a cycle that ends in the same state, therefore I'm guessing each ionization and recombination must be reversible processes.
Now recombinations do not always end up in the ground state. Then the recombination to a different level other than ground would lead to a cascade that would result in at a minimum of two photons, with at most only one of ionizing energy. Given enough time and repetition of recombining to a state other than ground there will eventually be no ionizing photon.
Since photons have zero chemical potential, this doesn't necessarily mean the system is in a different state does it? Just interestingly the system can no longer perform this process.
If this single atom case is reversible, are the same processes in a system of multiple atoms and photons reversible? This seems different since now the atoms can thermalize amongst each other. Imagine the gas is initially at a lower temperature than the photon gas, then given enough time the two gases will equilibrate. As before eventually there will be no more ionizing photons and you could not reverse the gas back to its original state of being colder than the photon gas. It seems the gas is then in a state which cannot be taken back to it's initial state.
If the system is both the photon and gas is this necessarily a different state? Did I pull a slight of hand, and the problem with this scenario is that the collisions were the irreversible process, not the ionization and recombinations themselves?