  The two dimensional layered transition metal dichalcogenides provide new
opportunities in future valley based information processing and also provide
ideal platform to study excitonic effects. At the center of various device
physics toward their possible electronic and optoelectronic applications is
understanding the dynamical evolution of various many particle electronic
states, especially exciton which dominates the optoelectronic response of TMDs,
under the novel context of valley degree of freedom. Here, we provide a brief
review of experimental advances in using helicity resolved ultrafast
spectroscopy, especially ultrafast pump-probe spectroscopy, to study the
dynamical evolution of valley related many particle electronic states in
semiconducting monolayer transitional metal dichalcogenides.
