  The optical emission spectra of semiconducting transition-metal
dichalcogenide monolayers highlight fascinating recombination processes of
charged excitons (trions). When charge tunable WSe2 monolayers are moderately
doped with electrons, a strong luminescence peak emerges just below the
well-understood spectral lines associated with the recombination of negatively
charged bright and dark trions. Despite previous investigations, its origin
remains elusive. Here, we demonstrate that this luminescence peak is the result
of electron-electron assisted recombination that brightens the dark trion
emission. Supporting evidence for this second-order recombination process comes
from identifying the equivalent brightened emission of positively charged dark
trions when the monolayer is electrostatically doped with holes. Remarkably,
the discovered hole-hole assisted luminescence peak emerges in the near
infrared, about 500 meV below the well-studied spectral region of excitons and
trions. In addition to identifying new recombination channels of these
excitonic complexes, our findings accurately determine the spin-split energies
of the conduction and valence bands. Both of which play crucial roles in
understanding the optical properties of WSe2 based homo- and hetero-structures.
