  We report transient optical signatures of the orbital-selective Mottness in
superconducting KxFe2-ySe2 crystals by using dual-color pump-probe
spectroscopy. Besides multi-exponential decay recovery dynamics of
photo-induced quasiparticles, a damped oscillatory component due to coherent
acoustic phonons emerges when the superconducting phase is suppressed by
increasing the temperature or excitation power. The oscillatory component
diminishes with significant enhancement of a slow decay component upon raising
temperature to 150-160 K. These results are in consistence with the picture of
orbital-selective Mott phase transition, indicating a vital role played by
electron correlation in the iron-based superconductors.
