  The molecular dark state participates in many important photon-induced
processes, yet is typically beyond the optical-spectroscopic measurement due to
the forbidden transition dictated by the selection rule. In this work, we
propose to use the scanning tunneling microscope (STM) as an incisive tool to
directly profile the dark-state transition density of a single molecule, taking
advantage of the localized static electronic field near the metal tip. The
detection of dark state is achieved by measuring the fluorescence from a higher
bright state to the ground state with assistant optical pumping. The current
proposal shall bring new methodology to study the single-molecule properties in
the electro-optical devices and the light-assisted biological processes.
