  Ultrafast time-resolved differential reflectivity of Bi2Se3 crystals is
studied using optical pump-probe spectroscopy. Three distinct relaxation
processes are found to contribute to the initial transient reflectivity
changes. The deduced relaxation timescale and the sign of the reflectivity
change suggest that electron-phonon interactions and defect-induced charge
trapping are the underlying mechanisms for the three processes. After the
crystal is exposed to air, the relative strength of these processes is altered
and becomes strongly dependent on the excitation photon energy.
