  We have investigated the exciton dynamics in transition metal dichalcogenide
mono-layers using time-resolved photoluminescence experiments performed with
optimized time-resolution. For MoSe2 monolayers, we measure
$\tau_{rad}=1.8\pm0.2$ ps that we interpret as the intrinsic radiative
recombination time. Similar values are found for WSe2 mono-layers. Our detailed
analysis suggests the following scenario: at low temperature (T $\leq$ 50 K),
the exciton oscillator strength is so large that the entire light can be
emitted before the time required for the establishment of a thermalized exciton
distribution. For higher lattice temperatures, the photoluminescence dynamics
is characterized by two regimes with very different characteristic times. First
the PL intensity drops drastically with a decay time in the range of the
picosecond driven by the escape of excitons from the radiative window due to
exciton- phonon interactions. Following this first non-thermal regime, a
thermalized exciton population is established gradually yielding longer
photoluminescence decay times in the nanosecond range. Both the exciton
effective radiative recombination and non-radiative recombination channels
including exciton-exciton annihilation control the latter. Finally the
temperature dependence of the measured exciton and trion dynamics indicates
that the two populations are not in thermodynamical equilibrium.
