The Jablonski Diagram
A Jablonski diagram illustrates the electronic states of a molecule as well as the transitions between them. These states are arranged vertically by energy and grouped horizontally by spin multiplicity. Nonradiative transitions are indicated by straight arrows and radiative transitions by squiggly arrows. The vibrational states of each electronic state are indicated with parallel horizontal lines.
For flow cytometry, its important to note that the energy of the emission is usually less than that of the absorption. As such, fluorescence normally occurs at lower energies or longer wavelengths.