  A scanning tunneling microscope is used to study the fluorescence of a model
charged molecule (quinacridone) adsorbed on a sodium chloride (NaCl)-covered
metallic sample. Fluorescence from the neutral and positively charged species
is reported and imaged using hyper-resolved fluorescence microscopy. A
many-body excitation model is established based on a detailed analysis of
voltage, current and spatial dependencies of the fluorescence and electron
transport features. This model reveals that quinacridone adopts a large palette
of charge states, transient or not, depending on the voltage used and the
nature of the underlying substrate. This model has a universal character and
explains the electronic and fluorescence properties of many other molecules
adsorbed on thin insulators.
