I've been wondering if it is thought energy transitions in an atom are instantaneous or not...as when a photon is absorbed or emitted. Whatever the answer, is the same case with transitions in molecules? Seems like creation and annihilation operators might describe what happens, but a quick scan of Wiki did not provide an obvious answer. It seems the center of a wave packet DOES suffer a delay when reflected from a potential discontinuity while there is no apparent delay with a classical particle. Is this a clue?