  We report charged exciton (trion) formation dynamics in doped monolayer
transition metal dichalcogenides (TMDs), specifically molybdenum diselenide
(MoSe2), using resonant two-color pump-probe spectroscopy. When resonantly
pumping the exciton transition, trions are generated on a picosecond timescale
through exciton-electron interaction. As the pump energy is tuned from the high
energy to low energy side of the inhomogeneously broadened exciton resonance,
the trion formation time increases by ~ 50%. This feature can be explained by
the existence of both localized and delocalized excitons in a disordered
potential and suggests the existence of an exciton mobility edge in TMDs. The
quasiparticle formation and conversion processes are important for interpreting
photoluminescence and photoconductivity in TMDs.
