Intervalley scattering in MoS$_2$ 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 MoS$_2$ 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 $\overline{K}$ followed by an ultrafast transfer ($< 50$~fs) to the conduction band minimum at $\overline{\Sigma}$. Both signals, at $\overline{K}$ and $\overline{\Sigma}$, 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. 
Robert Wallauer, Johannes Reimann, Nico Armbrust, Jens G\"udde and   Ulrich H\"ofer (Fachbereich Physik und Zentrum f\"ur Ma...
cond-mat.mes-hall