Direct determination of momentum-resolved electron transfer in the photoexcited van der Waals heterobilayer WS2/MoS2

Photoinduced charge separation in transition-metal dichalcogenide heterobilayers is being explored for moire excitons, spin-valley polarization, and quantum phases of excitons/electrons. While different momentum points can be critically involved in charge separation dynamics, little is known directly from experiments. Here we determine momentum-resolved electron dynamics in the WS2/MoS2 heterobilayer using time- and angle-resolved photoemission spectroscopy. Upon photoexcitation in the K valleys, we detect electrons in M/2, M, and Q valleys/points on timescales as short as similar to 70 fs, followed by dynamic equilibration in K and Q valleys in similar to 400 fs. These findings reveal the essential role of phonon scattering, the coexistence of direct and indirect interlayer excitons, and constraints on spin-valley polarization.