  Charged excitons called trions play an important role in the fundamental
valley dynamics in the newly emerging 2D semiconductor materials. We used
ultrafast pump- probe spectroscopy to study the valley trion dynamics in a
MoSe$_2$ monolayer grown by using chemical vapor deposition. The dynamics
display an ultrafast trion formation followed by a non-exponential decay. The
measurements at different pump fluences show that the trion decay dynamics
become slower as the excitation density increases. The observed trion dynamics
and the associated density dependence are a result of the trapping by two
defect states as being the dominating decay mechanism. The simulation based on
a set of rate equations reproduces the experimental data for different pump
fluences. Our results reveal the important trion dynamics and identify the
trapping by defect states as the primary trion decay mechanism in monolayer
MoSe$_2$ under the excitation densities used in our experiment.
