  We report ultrafast pump-probe spectroscopy examining exciton dynamics in
atomically thin MoS2. Spectrally- and temporally-resolved measurements are
performed to investigate the interaction dynamics of two important direct-gap
excitons (A and B) and their associated broadening kinetics. The two excitons
show strongly correlated inter-excitonic dynamic, in which the transient
blue-shifted excitonic absorption originates from the internal A-B excitonic
interaction. The observed complex spectral response is determined by the
exciton collision-induced linewidth broadening; the broadening of the B exciton
linewidth in turn lowers the peak spectral amplitude of the A exciton. Resonant
excitation at the B exciton energy reveals that inter-excitonic scattering
plays a more important role in determining the broadening kinetics than
free-carrier scattering.
