How does Fluorescence work?

When light waves hit the molecule, a photon is absorbed and gives the system energy, which promotes an electron from the ground state (S0) to an excited state (Sn). There are a few processes the molecule can then undergo (Figure 2), but the ones relevant here are vibrational relaxation, internal conversion and, of course, fluorescence.

Figure 2: A simplified Jablonski diagram showing the energy levels in a molecule and the different energy processes involved for fluorescence to occur.

Vibrational relaxation

Within each energy state, there are further, smaller energy levels called vibrational levels. The electron will drop between these from wherever it was promoted until it is at the lowest vibrational energy level within that excited state (v = 0). This drop is called vibrational relaxation and it emits a small amount of energy as heat energy.

Internal conversion

When at the lowest vibrational energy level of that state, the electron will need to drop to the next energy state. If in any excited state except the first one (ie Sn+1), it will do this by moving from a low vibrational energy level of the higher excited state to a high vibrational energy level of the excited state below it which sits at the same energy value. This is an isoenergetic process, meaning no energy is lost or gained.

Fluorescence

When, by vibrational relaxation and internal conversion, the electron reaches the lowest vibrational level of the first excited state (S1, v = 0), fluorescence will occur as the electron drops down to the ground state. The remaining energy will be emitted in the form of a photon of light. Due to the previous vibrational relaxation, this energy is not the same amount of energy that was originally absorbed, but lower. This lower energy means that the photon emitted will have a lower frequency and higher wavelength (Figure 3).

Figure 3: An electromagnetic spectrum with wavelength and frequency ranges for each type of light. As the equation shows, energy is indirectly proportional to wavelength so when the emitted light is at a lower energy it is at a higher wavelength. This higher wavelength means the emitted light is in the visible spectrum and so we can see it.