  One of the main tasks in the investigation of 2-dimensional transition metal
dichalcogenides is the determination of valley lifetimes. In this work, we
combine time-resolved Kerr rotation with electrical transport measurements to
explore the gate-dependent valley lifetimes of free conduction band electrons
of monolayer WSe$_2$. When tuning the Fermi energy into the conduction band we
observe a strong decrease of the respective valley lifetimes which is
consistent with both spin-orbit and electron-phonon scattering. We explain the
formation of a valley polarization by the scattering of optically excited
valley polarized bright trions into dark states by intervalley scattering.
Furthermore, we show that the conventional time-resolved Kerr rotation
measurement scheme has to be modified to account for photo-induced gate
screening effects. Disregarding this adaptation can lead to erroneous
conclusions drawn from gate-dependent optical measurements and can completely
mask the true gate-dependent valley dynamics.
