  Photoluminescence experiments from monolayer transition-metal dichalcogenides
often show that the binding energy of trions is conspicuously similar to the
energy of optical phonons. This enigmatic coincidence calls into question
whether phonons are involved in the radiative recombination process. We address
this problem, unraveling an intriguing optical transition mechanism. Its
initial state is a localized charge (electron or hole) and delocalized exciton.
The final state is the localized charge, phonon and photon. In between, the
intermediate state of the system is a virtual trion formed when the localized
charge captures the exciton through emission of the phonon. We analyze the
difference between radiative recombinations that involve real and virtual
trions (i.e., with and without a phonon), providing useful ways to distinguish
between the two in experiment.
