  Ultrafast terahertz (THz) pump{probe spectroscopy reveals unusual
out-of-equilibrium Cooper pair dynamics driven by femtosecond (fs) optical
quench of superconductivity (SC) in iron pnictides. We observe a two{step
quench of the SC gap, where an abnormally slow (many 100's of ps) quench
process is clearly distinguished from the usual fast (sub-ps)
hot{phonon{mediated scattering channel. This pair breaking dynamics depends
strongly on doping, pump uence, and temperature. The above observations,
together with quantum kinetic modeling of non-equilibrium SC and magnetic
correlations, provide evidence for photogeneration of a transient state where
SC competes with build{up of spin-density-wave (SDW) excitonic correlation
between quasi-particles (QP).
