Electronic intraband scattering in a transition-metal dichalcogenide observed by double-excitation ultrafast electron diffraction

Electronic dynamics in the excited state of transition-metal dichalcogenides (TMDs) has attracted great interest. To understand the ultrafast intraband scattering process of excited electrons in the conduction band, we demonstrated ultrafast time-resolved electron diffraction measurements with double-optical-pulse excitation and ultrafast transient reflectivity measurements of a TMD material, 2H-MoTe2. Due to the saturable absorption (or Pauli blocking) effect present in 2H-MoTe2, the system does not absorb the second excitation pulse until the excited electrons generated by the first excitation pulse with a specific fluence are scattered in the conduction band. By exploiting the Pauli blocking effect in ultrafast time-resolved electron diffraction measurements with double-optical-pulse excitation, we found that the excited electrons were scattered within 100 fs comparable to the excitation optical pulse duration. Furthermore, the excited electrons were relaxed to the lowest energy level of the conduction band (K- or Sigma-valley) within 1-2 ps.