Nuclear γ resonance time-domain interferometry: Quantum beat and radiative coupling regimes compared in revealing quasielastic scattering

Nuclear γ resonance time-domain interferometry (TDI) is a method where the interference pattern is built in time during the delayed scattering of synchrotron radiation by a system composed of two nuclear resonant targets and a nonresonant sample placed in between. The radiation transmitted through the upstream target is scattered by the sample at a finite angle and is transmitted through the downstream target. Atomic motions in the sample perturbing the delayed radiation can be revealed directly in the time interference pattern. The unique sharpness of nuclear resonant scattering allows one to investigate atomic motions proceeding for times in the range of nanoseconds to microseconds. The radiative coupling (RC) regime of TDI where the radiation from the upstream nuclear target is in resonance with the downstream target was investigated experimentally and compared with the quantum beat (QB) regime where the resonances in the targets are well separated. Clear evolutions of the interference patterns were observed with glycerol as a sample in both regimes, manifesting the increase of quasielastic scattering both with increasing temperature and with momentum transfer. However, the increase of quasielastic scattering is revealed in quite different ways: in the QB regime through pronounced changes of the quantum beat modulation of a fixed interference pattern, in the RC regime via strong changes of the interference pattern itself, mainly of its dynamical beat structure. It was possible to find relaxation parameters by which the two sets of completely different time evolutions for the QB and RC regimes were consistently fitted. Such a treatment will in the future considerably enlarge the dynamic range of the method and increase the reliability of the data analysis.