  We present studies of the photoexcited quasiparticle dynamics in MgB2 where,
using femtosecond optical techniques, Cooper pair breaking dynamics (PBD) have
been temporally resolved for the first time. The PBD are strongly temperature
and photoexcitation intensity dependent. Analysis of the PBD using the
Rothwarf-Taylor equations suggests that the anomalous PBD arises from the fact
that in MgB2 photoexcitation is initially followed by energy relaxation to high
frequency phonons instead of, as commonly assumed, e-e thermalization.
Furthermore, the bare quasiparticle recombination rate and the probability for
pair-breaking by phonons have been determined.
