  The newly developed two-dimensional layered materials provide perfect
platform for valley-spintronics exploration. To determine the prospect of
utilizing the valley degree of freedom, it is of great importance to directly
detect and understand the valley dynamics in these materials. Here, the exciton
valley dynamics in monolayer WSe$_2$ is investigated by the two-color
pump-probe magneto-optical Kerr technique. By tuning the probe photon energy in
resonance with the free excitons and trions, the valley relaxation time of
different excitonic states in monolayer WSe$_2$ is determined. Valley
relaxation time of the free exciton in monolayer WSe$_2$ is confirmed to be
several picoseconds. A slow valley polarization relaxation process is observed
to be associated with the trions, showing that the valley lifetime for trions
is one order of magnitude longer than that of free excitons. This finding
suggests that trion can be a good candidate for valleytronics application.
