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We investigate the quasiparticle relaxation and low-energy electronicstructure in undoped SmFeAsO and near-optimally doped SmFeAsO_{0.8}F_{0.2}single crystals - exhibiting spin-density wave (SDW) ordering andsuperconductivity respectively - using pump-probe femtosecond spectroscopy. Inthe undoped single crystals a single relaxation process is observed, showing aremarkable critical slowing down of the QP relaxation dynamics at the SDWtransition temperature T_{SDW}\simeq125{K}. In the superconducting (SC)crystals multiple relaxation processes are present, with distinct SC statequasiparticle recombination dynamics exhibiting a BCS-like T-dependentsuperconducting gap, and a pseudogap (PG)-like feature with an onset above 180Kindicating the existence of a pseudogap of magnitude2\Delta_{\mathrm{PG}}\simeq120 meV above T_{\mathrm{c}}. From the pump-photonenergy dependence we conclude that the SC state and PG relaxation channels areindependent, implying the presence of two separate electronic subsystems. Wediscuss the data in terms of spatial inhomogeneity and multi-band scenarios,finding that the latter is more consistent with the present data.

Quasiparticle relaxation dynamics in spin-density-wave and superconducting SmFeAsO_{1-x}F_{x} single crystals