  We investigate the dynamical formation of excitons from photoexcited
electron-hole plasma and its subsequent decay dynamics in monolayer MoS2 grown
by chemical vapor deposition using ultrafast pump and terahertz probe
spectroscopy. Different photoexcited electron-hole states are resolved based on
their distinct responses to THz photon and decay lifetime. The observed
transient THz transmission can be fit with two decay components: a fast
component with decay lifetime of 20 ps, which is attributed to exciton life
time including the exciton formation and subsequent intraexciton relaxation; a
slow component with extremely long decay lifetime of several ns due to either
localized exciton state or a long live dark exciton state which is uncovered
for the first time. The relaxation dynamics is further verified by temperature
and pump fluence dependent studies of the decay time constants.
