  Monolayer transition metal dichalcogenides offer the possibility of optical
control of the valley degree of freedom. In order to asses the potential of
these materials in applications, detailed knowledge of the valley dynamics is
essential. In this work, we apply low temperature time-resolved
photoluminescence (PL) measurements to investigate exciton valley relaxation
dynamics and, in particular, its behavior under strong excitation. At the
lowest excitation powers the inter valley scattering time is $\simeq 50$ ps,
but shortens by more than a factor of two at the highest powers. We attribute
this acceleration to either heating of the exciton system or the presence of a
dense exciton gas, which could influence the exciton valley properties.
Furthermore, we analyze the PL dynamics of excitons and trions. We find that
the PL decays for all peaks are bi-exponential and approximately independent of
the excitation power. We attribute the short decay to radiative recombination
and escape to a reservoir of dark states. The long decay is ascribed to a
transfer of excitons back from the reservoir. For the first time, we evaluate
the exciton PL decay time of $\simeq$ 10 ps. The latter process is
valley-conserving and occurs on a timescale of $\simeq$ 50 ps.
