We exploit time- and angle-resolved photoemission spectroscopy to determine the evolution of the out-of-equilibrium electronic structure of the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. The response of the Fermi-Dirac distribution to ultrashort IR laser pulses has been studied by modeling the dynamics of hot electrons after optical excitation. We disentangle a large increase in the effective temperature ($T^{*}$) from a shift of the chemical potential (${\mu }^{*}$), which is consequence of the ultrafast photodoping of the conduction band. The relaxation dynamics of $T^{*}$ and ${\mu }^{*}$ are $k$ independent and these two quantities uniquely define the evolution of the excited charge population. We observe that the energy dependence of the nonequilibrium charge population is solely determined by the analytical form of the effective Fermi-Dirac distribution.

• Received 11 September 2012

DOI:https://doi.org/10.1103/PhysRevB.86.205133