Role of Nonequilibrium Populations in Dark-Exciton Formation

The optical excitation of a bright exciton may be followed by the formation of lower-energy dark states. In these formation and relaxation processes, nonequilibrium exciton and phonon populations play a dominant role but remain so far largely unexplored, as most states are inaccessible by regular spectroscopies. Here, on the example of homobilayer 2H-MoS_{2}, we realize direct access to the full exciton relaxation cascade from experiment and theory. We find distinct changes in the time-, energy-, and in-plane momentum-resolved photoemission spectral function that can be explained only when considering the formation and subsequent thermalization of excitonic nonequilibrium occupation distributions. In agreement with microscopic many-particle calculations, we quantify the timescales for the formation of a nonequilibrium dark-excitonic occupation and its subsequent thermalization to 85 and 150 fs, respectively. Our results provide a previously inaccessible view of the complete exciton relaxation cascade, which is of importance for the future characterization of nonequilibrium excitonic phases and the efficient design of optoelectronic devices.