I hope someone can give me some new insight to understand this.
Fermi's golden rule is wildly used to calculate the rate for RET. I have some difficulties in understanding its justification and connected to this its explicit application.
Fermi's rule can be applied to calculate an irreversible decay rate. My first question would be how this is given in RET. After the transition we end up with a similar state as the initial where the acceptor is excited and the donor is in its ground state and the electromagentic field returns to its vacuum state. This suggests that the process is reversible and we should consider some dynamic, i.e. oscillations.
I think connected to this is my second problem. We find that the transition rate according to Fermi's rule is proportional to some density of final states. What is this density supposed to be in the case of RET, where the atomic system is described by discrete states and the electro-magnetic field has neither in its initial nor in its final state any excitation.
I often find Fermi's rule for discrete systems as proportional to the delta-distribution, which is just not something that can be used for calculating a finite rate and has no interpretation, in my opinion. Only by introducing some continuum we end up with a finite result, as e.g. seen in the question posed here: Fermi's Golden Rule and Density of States
