  We have examined the relaxation of photoinduced quasiparticles in the
heavy-fermion superconductor PuCoGa5. The deduced electron-phonon coupling
constant is incompatible with the measured superconducting transition
temperature Tc=18.5 K, which speaks against phonon-mediated superconducting
pairing. Upon lowering the temperature, we observe an order-of-magnitude
increase of the quasiparticle relaxation time in agreement with the phonon
bottleneck scenario - evidence for the presence of a hybridization gap in the
electronic density of states. The modification of photoinduced reflectance in
the superconducting state is consistent with the heavy character of the
quasiparticles that participate in Cooper pairing.
