LAUSR

Photoemitted electrons reveal large-momentum (“dark”) excitons in monolayer WSe2 Two-dimensional (2D) semiconductors, such as transition-metal dichalcogenides, may enable new optoelectronic technologies (1). The optical excitation in these atomically thin materials creates tightly bound excitons composed of an excited electron and a valence-state hole (2), as well as a plethora of exciton complexes due to the reduced screening in Coulomb attraction (3–5). So far, excitons with large momenta have not been directly probed because photons only carry very small momenta and cannot directly interact with large-momentum excitons, but these dark excitons are predicted to exist in certain 2D semiconductors (6, 7). On page 1199 of this issue, Madéo et al. (8) used time- and angle-resolved photoemission spectroscopy (TR-ARPES) to directly probe dark excitons in monolayer tungsten diselenide (WSe2). By tracking the dynamics of electrons that constitute both bright and dark excitons, the authors reveal how both are formed and show that the latter outnumber the former at steady state.