  Excitons with binding energies of a few hundreds of meV control the optical
properties of transition metal dichalcogenide monolayers. Knowledge of the fine
structure of these excitons is therefore essential to understand the
optoelectronic properties of these 2D materials. Here we measure the exciton
fine structure of MoS2 and MoSe2 monolayers encapsulated in boron nitride by
magneto-photoluminescence spectroscopy in magnetic fields up to 30 T. The
experiments performed in transverse magnetic field reveal a brightening of the
spin-forbidden dark excitons in MoS2 monolayer: we find that the dark excitons
appear at 14 meV below the bright ones. Measurements performed in tilted
magnetic field provide a conceivable description of the neutral exciton fine
structure. The experimental results are in agreement with a model taking into
account the effect of the exchange interaction on both the bright and dark
exciton states as well as the interaction with the magnetic field.
