  Femtosecond pump-probe studies show that carrier dynamics in MgB2 films is
governed by the sub-ps electron-phonon (e-ph) relaxation present at all
temperatures, the few-ps e-ph process well pronounced below 70 K, and the
sub-ns superconducting relaxation below Tc. The amplitude of the
superconducting component versus temperature follows the superposition of the
isotropic dirty gap and the 3-dimensional gap dependences, closing at two
different Tc values. The time constant of the few-ps relaxation exhibits a
double-divergence at temperatures corresponding to the Tc's of the two gaps.
