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Photo-induced charge separation in transition metal dichalcogenide heterobilayers is being explored for moiré excitons, spin-valley polarization, and quantum phases of excitons/electrons. While different momentum spaces may 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 (TR-ARPES). Upon photoexcitation in the K valleys, we detect electrons in M/2, M, and Q valleys/points on time scales as short as ~70 fs, followed by dynamic equilibration in K and Q valleys in ~400 fs. The interlayer charge transfer is accompanied by momentum-specific band renormalization. These findings reveal the essential role of phonon scattering, the coexistence of direct and indirect interlayer excitons, and constraints on spin-valley polarization. Monolayer transition metal dichalchogenides (TMDCs) are excellent models for the exploration of semiconductor physics at the two-dimensional (2D) limit, with potential applications in electronics, optoelectronics, and quantum devices [1]. Heterobilayers formed from stacking two TMDC monolayers may bring about new physical properties, such as moire excitons [2,3], long-lived spin valley polarization [4,5] and exciton condensation [6]. For TMDC heterobilayers, photoexcitation of one monolayer leads to electron or hole transfer to the other layer, resulting in interlayer excitons with electron and hole spatially separated across the interface [7–13]. Past experiments indicated that interlayer charge transfer occurs on ultrafast (<100 fs) time scales, regardless of relative layer orientations [11,12,14,15]. This observation is surprising since the conduction band minimum (CBM) and valence band maximum (VBM) are * To whom correspondence should be addressed. E-mail: xyzhu@columbia.edu

DIRECT DETERMINATION OF MOMENTUM-RESOLVED ELECTRON TRANSFER IN PHOTOEXCITED MoS2/WS2 VAN DER WAALS HETEROBILAYER