  Time- and angle-resolved photoemission spectroscopy (trARPES) is a powerful
spectroscopic method to measure the ultrafast electron dynamics directly in
momentum-space. However, band gap materials with exceptional strong Coulomb
interaction such as monolayer transition metal dichlacogenides (TMDC) exhibit
tightly bound excitons, which dominate their optical properties. This rises the
question whether excitons, in particular their formation and relaxation
dynamics, can be detected in photoemission. Here, we develope a fully
microscopic theory of the temporal dynamics of excitonic time- and angle
resolved photoemission with particular focus on the phonon-mediated
thermalization of optically excited excitons to momentum-forbidden dark exciton
states. We find that trARPES is able to probe the ultrafast exciton formation
and relaxation throughout the Brillouin zone.
