Photoluminescence is a process where a molecule absorbs light energy and re-emits it in the form of light. This phenomenon occurs when a substance absorbs photons, promoting its electrons to higher energy level excited states, followed by a relaxation process in which the electrons return to their original ground state energy levels and emit light. Photoluminescence is widely observed in various materials, including semiconductors, and organic and inorganic compounds.

A pair of electrons in a singlet spin state occupy the same electronic ground state with opposite spins, while a triplet excited state occurs when an electron's spin is no longer paired with that of the ground state. There are two main types of photoluminescence based on the electron spin states involved: fluorescence and phosphorescence.

Fluorescence is a type of photoluminescence characterized by rapid decay time, typically ranging from nanoseconds to microseconds. In fluorescence, the excited and ground states have the same electron spin multiplicity, meaning that the electron spin remains unchanged during the transition. The process involves singlet-singlet transitions, where both the excited and ground states are singlet states where all electrons are paired.

Phosphorescence is another type of photoluminescence characterized by significantly longer decay times, extending from milliseconds to minutes. In phosphorescence, the excited and ground states have different electron spin multiplicities. The process involves triplet-singlet transitions, where the excited state is a triplet state (two unpaired electrons with parallel spins), and the ground state is a singlet state. These transitions are 'spin-forbidden,' meaning the electron spin must change during the transition.

Both fluorescence and phosphorescence can be used in various applications, such as optical sensors, bioimaging, and organic light-emitting diodes. Photoluminescence spectra are recorded by measuring the intensity of emitted radiation in relation to either the excitation wavelength or the emission wavelength. Excitation spectra are obtained by monitoring emission at a fixed wavelength while varying the excitation wavelengths. Emission spectra are obtained by using a fixed wavelength to excite molecules.