We present studies of the photoexcited quasiparticle dynamics in ${\mathrm{M}\mathrm{g}\mathrm{B}}_2$ 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 ${\mathrm{M}\mathrm{g}\mathrm{B}}_2$ photoexcitation is initially followed by energy relaxation to high frequency phonons instead of, as commonly assumed, $e\mathrm{\text{−}}e$ thermalization. Furthermore, the bare quasiparticle recombination rate and the probability for pair breaking by phonons have been determined.

• Received 16 May 2003

DOI:https://doi.org/10.1103/PhysRevLett.91.267002