Momentum-resolved observation of ultrafast interlayer charge transfer between the topmost layers of MoS2

How fast is the charge transfer between two layers of transition metal dichalcogenides and where does it take place in momentum space? Two-photon photoemission using high-harmonic probe pulses can answer this question, as the authors demonstrate here for the topmost layers of MoS${}₂$. By tuning pump pulses below the topmost-layer gap, they excite electrons in deeper layers and probe only the topmost layer. GW-based tight-binding calculations support the experimental findings and explain why the electron transfer takes place at $\overline{\mathrm{\ensuremath{\Sigma}}}$.