Intervalley scattering in MoS2 imaged by two-photon photoemission with a high-harmonic probe

We report on the direct mapping of electron transfer in the momentum space of bulk MoS2 by means of time-and angle-resolved two-photon photoemission with a high-harmonic probe. For this purpose, we have combined a high-repetition rate high-harmonic source with tunable femtosecond pump pulses and a 3D (k(x); k(y); E) electron spectrometer. We show that optical excitation slightly above the A exciton resonance results in an immediate occupation of the conduction band at (K) over bar followed by an ultrafast transfer (<50 fs) to the conduction band minimum at <(Sigma)over bar>. Both signals, at (K) over bar and (Sigma) over bar, do not vanish over the observed period of 400 fs. The technique described here enables direct access to the charge transfer dynamics in k-space and allows the study of decay times and decay channels in various systems with dependence on the excess energy or helicity of the excitation. Published by AIP Publishing.