Effects of Light on Atoms and Molecules - Fluorescence and Phosphorescence

There are two sub-groups of photoluminescence, fluorescence and phosphorescence. They differ in the quantum state transitions that lead to the luminescence and thereby in the duration of their photon re-emission. Fluorescence ends nanoseconds after the excitation, whereas phosphorescence can last for seconds or in some cases even for hours. Quantum states cannot only be divided into excited and ground states but also into singlet and triplet states which are based on different spin alignments.

The spin is a quantum mechanical property of elementary particles. In simplified terms, it describes the intrinsic angular momentum of a particle, e.g. an electron, caused by its rotation and is characterized via the spin quantum number "s". There are two possible orientations for the spin of an electron, s = +1/2 (positive) or s = -1/2 (negative).

Spin pairs can either be combined parallel (both spins are either positive or negative) or antiparallel (one spin is positive, the other is negative) in their orientation to each other. If the electrons occupy the same atomic orbital, their spins will have to be antiparallel. In antiparallel spin pairs the individual angular momentums compensate each other and the total angular momentum gets a value of zero. This spin alignment is called singlet state. But if two electrons occupy two different orbitals, their spins may be parallel to each other. Two parallel spins do not compensate and get a value different from zero. In this case the spins are said to be in a triplet state.

Fluorescence describes the emission of a photon while an electron transits from an excited singlet state to another singlet state of lower energy, typically the ground state. This transition occurs rapidly after the excitation. Energy is released as a photon. The wavelength of the emitted photon is of greater wavelength than the exciting light, as the emission occurs after the fluorophore has released some of the energy to the surroundings. This phenomenon was discovered by George Gabriel Stokes in 1852 and is named Stokes shift.

Phosphorescence is very similar to fluorescence but it depends on the formation of excited triplet states. Excited triplet states can be formed in some molecules. The paired spins can be uncoupled in a process called intersystem crossing. The intersystem crossing is influenced by the spin-orbit coupling. This involves the interaction of the magnetic moment of an electron due to its spin and the magnetic moment due to its orbital angular momentum.

In the triplet state, the electron releases energy to its surroundings and reaches the triplet ground state. This state is of higher energy than the ground state but also of lower energy than the excited singlet state. The electron cannot switch back to the singlet state and cannot transit to the ground state as the spins are parallel and the transition is now spin-forbidden. But a few changes from the triplet to the ground state are possible at a time. These events of intersystem crossing give rise to the emission of photons. Phosphorescence therefore describes the photon emission during the transition from a triplet state to the ground state.