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Atomically thin layered van der Waals heterostructures feature exotic andemergent optoelectronic properties. With growing interest in these novelquantum materials, the microscopic understanding of fundamental interfacialcoupling mechanisms is of capital importance. Here, using multidimensionalphotoemission spectroscopy, we provide a layer- and momentum-resolved view onultrafast interlayer electron and energy transfer in amonolayer-WSe$_2$/graphene heterostructure. Depending on the nature of theoptically prepared state, we find the different dominating transfer mechanisms:while electron injection from graphene to WSe$_2$ is observed afterphotoexcitation of quasi-free hot carriers in the graphene layer, we establishan interfacial Meitner-Auger energy transfer process following the excitationof excitons in WSe$_2$. By analysing the time-energy-momentum distributions ofexcited-state carriers with a rate-equation model, we distinguish these twotypes of interfacial dynamics and identify the ultrafast conversion of excitonsin WSe$_2$ to valence band transitions in graphene. Microscopic calculationsfind interfacial dipole-monopole coupling underlying the Meitner-Auger energytransfer to dominate over conventional F\"orster- and Dexter-type interactions,in agreement with the experimental observations. The energy transfer mechanismrevealed here might enable new hot-carrier-based device concepts with van derWaals heterostructures.

Observation of ultrafast interfacial Meitner-Auger energy transfer in a van der Waals heterostructure