  We present the first femtosecond studies of electron-phonon (e-ph)
thermalization in heavy fermion compounds. The e-ph thermalization time
tau_{ep} increases below the Kondo temperature by more than two orders of
magnitude as T = 0 K is approached. Analysis using the two-temperature model
and numerical simulations based on Boltzmann's equations suggest that this
anomalous slowing down of the e-ph thermalization derives from the large
electronic specific heat and the suppresion of scattering between heavy
electrons and phonons.
