Momentum-Resolved Exciton Coupling and Valley Polarization Dynamics in Monolayer WS2

Using time- and angle-resolved photoemission, we present momentum- and energy-resolved measurements of exciton coupling in monolayer WS2. We observe strong intravalley coupling between the B1s exciton and An>1 states. Our measurements indicate that the dominant valley depolarization mechanism conserves the exciton binding energy and momentum. While this conservation is consistent with Coulomb exchange-driven valley depolarization, we do not observe a momentum or energy dependence to the depolarization rate as would be expected for the exchange-based mechanism.