  Excitons dominate the optical properties of monolayer transition metal
dichalcogenides (TMDs). Besides optically accessible bright exciton states,
TMDs exhibit also a multitude of optically forbidden dark excitons. Here, we
show that efficient exciton-phonon scattering couples bright and dark states
and gives rise to an asymmetric excitonic line shape. The observed asymmetry
can be traced back to phonon-induced sidebands that are accompanied by a
polaron redshift. We present a joint theory-experiment study investigating the
microscopic origin of these sidebands in different TMD materials taking into
account intra- and intervalley scattering channels opened by optical and
acoustic phonons. The gained insights contribute to a better understanding of
the optical fingerprint of these technologically promising nanomaterials.
