It is therefore desired to identify well defined bosonic or electronic signatures in time-resolved spectroscopy. In this talk results of femtosecond time- and angle-resolved photoemission on cuprate and Fe-based superconductors will be presented. Experimentally observed, well defined boson signatures [4,5], which originate from restrictions in the relaxation phase space  will be discussed. We show that coupling to specific bosonic excitations, which were identified by ultrafast electron diffraction for the case of Bi-2212 to consist of in-plane lattice vibration,  inhibits thermalization of the excited electron distribution. Such microscopic insight provides opportunities to analyze the electron-boson coupling directly, without the assumption of a thermalized electron distribution.
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