Paramagnetic relaxation as seen by nuclear resonant forward scattering of synchrotron radiation

Nuclear forward scattering of synchrotron radiation, which can be regarded as M\"ossbauer spectroscopy in the time domain, is a very promising tool for the investigation of dynamic phenomena. The interpretation of the resulting time-dependent intensity of the scattered radiation can be achieved by incorporating the stochastic theory of line shape into the program package SYNFOS, the theoretical basis of which has been described in an earlier paper [Haas et al., Phys. Rev. B 56, 14 082 (1997)]. This is demonstrated by means of measurements carried out on the paramagnetic iron porphyrin complex $[{\mathrm{F}\mathrm{e}(\mathrm{C}\mathrm{H}}_{3}{\mathrm{C}\mathrm{O}\mathrm{O})(\mathrm{T}\mathrm{P}}_{\mathrm{piv}}\mathrm{P}){]}^{\ensuremath{-}}$ that exhibits dynamic effects due to spin-lattice relaxation at temperatures around 10 K.