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The 1 s  exciton—the ground state of a bound electron-hole pair—is central to understanding the photoresponse of monolayer transition metal dichalcogenides. Above the 1 s  exciton, recent visible and near-infrared investigations have revealed that the excited excitons are much richer, exhibiting a series of Rydberg-like states. A natural question is then how the internal excitonic transitions are interrelated on photoexcitation. Accessing these intraexcitonic transitions, however, demands a fundamentally different experimental tool capable of probing optical transitions from 1 s  ‘bright' to  np  ‘dark' states. Here we employ ultrafast mid-infrared spectroscopy to explore the 1 s  intraexcitonic transitions in monolayer MoS 2 . We observed twofold 1 s →3 p  intraexcitonic transitions within the A and B excitons and 1 s →2 p  transition between the A and B excitons. Our results revealed that it takes about 0.7 ps for the 1 s  A exciton to reach quasi-equilibrium; a characteristic time that is associated with a rapid population transfer from the 1 s  B exciton, providing rich characteristics of many-body exciton dynamics in two-dimensional materials.

1s-intraexcitonic dynamics in monolayer MoS2 probed by ultrafast mid-infrared spectroscopy