  Single layers of transition metal dichalcogenides (TMDs) are direct gap
semiconductors with nondegenerate valley indices. An intriguing possibility for
these materials is the use of their valley index as an alternate state
variable. Several limitations to such a utility include strong, phonon-enabled
intervalley scattering, as well as multiparticle interactions leading to
multiple emission channels. We prepare single-layer WS$_{2}$ such that the
photoluminescence is from either the neutral or charged exciton (trion). After
excitation with circularly polarized light, the neutral exciton emission has
zero polarization, however, the trion emission has a large polarization (28%)
at room temperature. The trion emission also has a unique, non-monotonic
temperature dependence that we show is a consequence of the multiparticle
nature of the trion. This temperature dependence enables us to determine that
coulomb assisted intervalley scattering, electron-hole radiative recombination,
and a 3-particle Auger process are the dominant mechanisms at work in this
system. Because this dependence involves trion systems, one can use gate
voltages to modulate the polarization (or intensity) emitted from TMD
structures.
