In monolayer MoSe2 strong photoluminescence from excitons and trions, resembling hydrogen-atom and hydrogen-ion like quasiparticles, can be observed after photoexcitation. We show that THz excitation with a photon energy in resonance or above the trion binding energy results in photo-dissociation of trions into excitons and free electrons. Utilizing this process, the photoluminescence wavelength can be switched on ps timescales in a controllable manner. The experiments pave the way for studies of increasingly complex Coulomb-bound many-body systems and offer perspectives for device applications combining THz and visible photonics.