Radiative decoupling and coupling of nuclear oscillators by stepwise Doppler-energy shifts

The nuclear forward scattering of synchrotron radiation ~NFSSR! of two spatially separated but otherwise identical stainless-steel targets is strongly influenced by the application of approximately stepwise Doppler energy shifts to one of the targets in synchronization with the synchrotron radiation ~SR! pulse. The decisive role of radiative coupling in NFSSR, i.e., the influence of the radiation field emitted by the upstream target on the oscillators of the downstream target, has been clarified in decoupling and coupling experiments. In decoupling experiments, radiative coupling is initially strong and becomes drastically reduced when the Doppler motion is started some time interval after the prompt excitation of the two-target system by the SR pulse. In addition, due to the Doppler energy shift a pronounced quantum beat ~QB! develops. Depending on the starting time of the Doppler motion, the onset of the QB manifests itself as a rapid decrease or increase of the NFSSR intensity. In coupling experiments, the Doppler motion is started before the SR pulse arrives and a QB is observed right after the SR excitation. Some time interval thereafter the motion is stopped, so that the resonance energies of both targets coincide. As a consequence, the QB disappears and radiative coupling can develop. Depending on the time when the Doppler motion is stopped, this coupling can be responsible for the main contribution to the NFSSR intensity. The changes of the evolution due to decoupling or coupling also strongly depend on the details of the Doppler velocity profile. We show that precise velocity profiles on a nanosecond time scale can be derived by applying the method of quantum-beat analysis and independently by a fitting procedure based on the dynamical theory of scattering. All NFSSR evolutions investigated are interpreted and quantitatively described within the dynamical theory in terms of the interference of the wave fields reemitted by the scattering system.