  Scientific curiosity to uncover original optical properties and
functionalities of atomically thin semiconductors, stemming from unusual
Coulomb interactions in the two-dimensional geometry and multi-valley band
structure, drives the research on monolayers of transition metal
dichalcogenides (TMDs). While recent works ascertained the exotic energetic
schemes of exciton complexes in TMDs, we here employ four-wave mixing
microscopy to indicate that their subpicosecond dynamics is determined by the
surrounding disorder. Focusing on a monolayer WS$_2$, we observe that exciton
coherence is lost primarily due to interaction with phonons and relaxation
processes towards optically dark excitonic states. Notably, when temperature is
low and disorder weak excitons large coherence volume results in huge
oscillator strength, allowing to reach the regime of radiatively limited
dephasing and we observe long valley coherence. We thus elucidate the crucial
role of exciton environment in the TMDs on its dynamics and show that revealed
mechanisms are ubiquitous within that family.
