  We exploit time- and angle- resolved photoemission spectroscopy to determine
the evolution of the out-of-equilibrium electronic structure of the topological
insulator Bi2Se. The response of the Fermi-Dirac distribution to ultrashort IR
laser pulses has been studied by modelling the dynamics of the hot electrons
after optical excitation. We disentangle a large increase of 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 non-equilibrium charge population is solely determined by the
analytical form of the effective Fermi-Dirac distribution.
