Direct observation of ultrafast dark exciton formation in monolayer WS$_2$

The exciton landscape in transition metal dichalcogenides is crucial for their optical properties and difficult to measure experimentally, since many exciton states are not accessible to optical interband spectroscopy. Here, we combine a tunable pump, high-harmonic probe laser source with a 3D momentum imaging technique to map photoemitted electrons within the entire first Brillouin zone of monolayer WS$_2$. This gives us momentum-, energy- and time-resolved access to excited states that are forming during and after optical excitation on an ultrafast timescale. The high temporal resolution of our experimental setup allows us to follow the formation of a momentum-forbidden dark K$\Sigma$ exciton a few tens of femtoseconds after optical excitation. The experimental results are in excellent agreement to a fully microscopic theory, resolving the temporal and spectral dynamics of bright and dark excitons in WS$_2$.