  Atomically thin transition metal dichalcogenides (TMDs) are direct-gap
semiconductors with strong light-matter and Coulomb interaction. The latter
accounts for tightly bound excitons, which dominate the optical properties of
these technologically promising materials. Besides the optically accessible
bright excitons, these systems exhibit a variety of dark excitonic states. They
are not visible in optical spectra, but can strongly influence the coherence
lifetime and the linewidth of the emission from bright exciton states. In a
recent study, an experimental evidence for the existence of such dark states
has been demonstrated, as well as their strong impact on the quantum efficiency
of light emission in TMDs. Here, we reveal the microscopic origin of the
excitonic coherence lifetime in two representative TMD materials (WS$_2$ and
MoSe$_2$) within a joint study combining microscopic theory with optical
experiments. We show that the excitonic coherence lifetime is determined by
phonon-induced intra- and intervalley scattering into dark excitonic states.
Remarkably, and in accordance with the theoretical prediction, we find an
efficient exciton relaxation in WS$_2$ through phonon emission at all
temperatures.
