  We report on systematic excitation-density dependent all-optical femtosecond
time resolved study of the spin-density wave state in iron-based
superconductors. The destruction and recovery dynamics are measured by means of
the standard and a multi-pulse pump-probe technique. The experimental data are
analyzed and interpreted in the framework of an extended three temperature
model. The analysis suggests that the optical-phonons energy-relaxation plays
an important role in the recovery of almost exclusively electronically driven
spin density wave order.
