Figure 3.

A simplified Perrin-Jablonski energy level diagram for a fluorescent molecule. The arrows represent absorption of excitation photon energy causing the transition from the lowest vibrational levels of the ground state (S₀) to the excited state (S₁). Thermal energy is lost by internal conversion and the transition from the excited state to the ground state is always from the lowest level of S₁. The de-excitation transitions can occur by the emissive (k_f) pathway or by other competing non-emissive (k_nf) pathways.