Coherent Nuclear Resonance Fluorescence
For a long time, the impact of ideas with respect to coherency and scattering on the art and science of Mössbauer spectroscopy was not very essential. The main line of development of Mössbauer experiments rested in the frame of absorption spectroscopy. Mössbauer physicists dealt mostly with absorption spectra taken either in traditional transmission experiments or in measurements of the conversion electron yield. For the interpretation and description of these spectra, it was appropriate to use the picture of interaction of -quantum with an individual nucleus where the nuclear resonant absorption cross-section was applied. The coherent properties of radiation and those of the interaction mechanism were not explicitly involved in these studies. In the meantime, coherent phenomena with Mössbauer -rays were thoroughly investigated, starting soon after Mössbauer’s discovery. The present paper is aimed to illuminate this side of the Mössbauer story.
The best conditions under which the coherent radiative decay of nuclear exciton can be observed are realized by the pulsed excitation of the nuclear ensemble. This approach became possible with the creation of the powerful sources of synchrotron radiation (SR). The idea to apply SR to study nuclear fluorescence was put forward by Mössbauer [26, 27] and Ruby [28]. The first coherent response of a nuclear ensemble to an SR pulse was unequivocally observed by Gerdau et al. [29].