The word luminous basically means giving off light. Most objects in our world give off light because they are in possession of energy that originated from the sun, the most luminous object we know and that we can see. In contrast to the moon who seems to give off light, but is only reflecting it from the sun like a giant mirror made of rock.
Basically, there are 3 main forms of luminescence: fluorescence, phosphorescence and chemiluminescence. Two of these, namely fluorescence and phosphorescence, are forms of photoluminescence. The difference between photo- and chemiluminescence is that in photoluminescence the luminescence reaction is triggered by light whereas in chemiluminescence the light emission is triggered by a chemical reaction. The basis of both forms, fluorescence and phosphorescence, is the ability of a substance to absorb light and then emit this light at a longer wavelength which means lower energy, just the timescale in which that happens is different. While in fluorescent reactions the emission takes place immediately and is only visible as long as the light source is on (e.g. UV lights), in phosphorescent reactions the material can store the absorbed energy and release it later, resulting in an afterglow that persists after the light has been switched off. So, if it disappears immediately, it’s fluorescence. If it lingers, it’s phosphorescence. And if it needs chemical activation, it’s chemiluminescence.
As an example, you could imagine a night club where the fabric and teeth glow under the black light (fluorescence), the emergency exit sign glows (phosphorescence) and also the glow sticks glow (chemiluminescence).
But how exactly does it work in detail? Learn more in our main article about Luminescence.
Materials that produce light instantly are called fluorescent. The atoms inside them absorb energy and become “excited”. While returning to the normal state in approx. a hundred thousandth of a second (10-9 to 10-6 sec), they release the energy as tiny particles of light called photons.
Technically speaking, fluorescence is a radiative mechanism by which excited electrons transition from the lowest excited state (S1) to the ground state (S0). During this process the electron losses a bit of its energy by vibrational relaxation, resulting in the emitted photon having lower energy and therefore longer wavelength.
Looking at phosphorescence we need to take a short detour into electron spin to understand the differences between fluorescence and phosphorescence. Spin is a fundamental property of an electron and a form of angular momentum that defines behavior in an electromagnetic field. The spin can only have a value of ½ and an orientation of either up or down. An electron’s spin is therefore designated as +½ or -½, or alternatively as ↑ or↓. If electrons are on the same orbital of an atom, they always have an antiparallel spin at single ground state (S0). When promoted into an exited state, the electron maintains its spin orientation and a singlet excited state (S1) is formed, where the both spin orientations remain paired as antiparallel. All relaxation events in fluorescence are spin neutral and the spin orientation of the electron is maintained at all times.
In phosphorescence this is different. Here you have fast (10-11 to 10-6 sec) intersystem crossings from singlet excited state (S1) to an energetically favorable triplet excited state (T1). This leads to inversion of the electron spin and these states are characterized by parallel spin of both electrons and are metastable. Here the relaxation occurs by phosphorescence, resulting in another flip of the electron spin and the emission of a photon. The return to the relaxed singlet ground state (S0) might occur after a longer delay (10-3 to >100 sec). In this process more energy is consumed by non-radiative processes during phosphorescent relaxation than in fluorescence, leading to a higher energy difference between the absorbed and emitted photon and therefore a bigger shift in wavelength.