What is Fluorescence?

Fluorescence and luminescence in everyday vernacular are often used interchangeably, but fluorescence is a type of luminescence, specifically a type of photoluminescence. Other types of luminescence include phosphorescence, also a type of photoluminescence, and chemiluminescence. As the prefix photo- suggests, light is a key factor for photoluminescence, in this case, fluorescence, to occur. Simply put, a molecule can absorb a photon of light and therefore its energy, and then reemit a photon of lower energy and higher wavelength. The emitted light is fluorescence or fluorescent light.

Why does Fluorescence occur?

To understand why fluorescence occurs, a brief understanding of the quantum mechanical make-up of a molecule is needed. An atom is made up of a net positive nucleus, which holds in place negative electrons. The electrons are held in discrete energy levels. A system will always favour the lowest energy state and so when atoms combine to make molecules, they do so because this lowers their energy and makes them more stable. When molecules form, new energy levels, known as orbitals, are formed that the electrons can exist in. For each atom or molecule, the energy levels are certain and discrete, and the system is said to be quantised.

The lowest possible energy system, known as the ground state, is always favoured and so the system exists primarily in this state. When energy is put into the system, a molecule is said to be in an excited state and will quickly undergo processes that emit energy from the system to return it to the ground state. One such process is fluorescence.