(abbreviated as PL) is a process in which a substance absorbs photons (electromagnetic radiation) and then
mechanically, this can be described as an excitation to a higher energy state
and then a return to a lower energy
state accompanied by the emission of a photon. This is one of many forms of luminescence (light emission) and is
distinguished by photoexcitation (excitation by photons),
hence the prefix photo-. The period between absorption and emission is
typically extremely short, in the order of 10 nanoseconds.
Under special circumstances, however, this period can be extended into
minutes or hours.