Fluorescence is a form of luminescence (emission of light that is not associated with a rise in temperature) that results from the transition between the two lowest electronic states (the first excited state and the ground state), of the same nature. When a molecule is irradiated with ultraviolet and visible light, it might be temporarily raised from low energy and equilibrium state (ground state) to high energy and non-equilibrium state (excited state), where it has an excess of energy. The probability of finding this molecule in one possible excited state (almost all molecules encountered in our life exist in a singlet state, where all electrons in the molecule are spin paired) depends on the transition probabilities and the excitation wavelength. The Perrin-Jablonski diagram is a convenient way for resuming the physical processes occurring since the molecule absorbs light until it returns once again to the ground state, see Figure 1.

Being a non-equilibrium state, the amount of time that the molecule stays in S₁ is usually too short (around 10^-9 seconds). Depending on the structure of the excited molecule, a radiative transition from S₁ might occur by the spontaneous emission of a photon. This process is called fluorescence emission. The fluorescence emission spectrum is always found at higher wavelengths than the absorption spectrum because of the energy loss in the excited state due to a number of energy transfer processes.

Figure 1 – Simplified Perrin-Jablonski diagram describing the electronic levels of common organic molecules.