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.
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