Hi all. This is my first post @ physics forum. For last two weeks I have been wondering about this problem: We all know that an atom or molecule subjected to a time-dependent perturbation makes electronic transition, and whether the transition is allowed or not depends on the transition moment of the atom/molecule and the frequency of the external field. My question is, what is the work done for any particular electronic transition and what is the force on the electrons for that transition. I have tried using Ehrenfest theorem for that, but kinda clueless right now. Any insight will be greatly appreciated.