  Monolayers of transition metal dichalcogenides present an intriguing platform
to investigate the interplay of excitonic complexes in two dimensional
semiconductors. Here, we use optical spectroscopy to study light-matter
coupling and non-equilibrium relaxation dynamics of three-particle exciton
states, commonly known as trions. We identify the consequences of the exchange
interaction for the trion finestructure in tungsten-based monolayer materials
from variational calculations and experimentally determine the resulting
characteristic differences in their oscillator strength. It allows us to
quantitatively extract trion populations from time-resolved photoluminescence
measurements and monitor their dynamics after off-resonant optical injection.
At liquid helium temperature we observe pronounced non-equilibrium distribution
of the trions during their lifetime with comparatively slow equilibration that
occurs on time-scales up to several 100's of ps. In addition, we find an
intriguing regime of population inversion at lowest excitation densities that
builds up and is maintained during 10's of picoseconds. With increased lattice
temperature the equilibrium is established more rapidly and the inversion
disappears, highlighting the role of thermal activation for efficient
scattering between exchange-split trions.
