Editing Fluorescence (section)

=== Mechanism ===
[[File:Ruby ball fluorescence @ 520nm laser illumination.jpg|thumb|A ruby [[ball lens]] atop a green laser-pointer. The green beam [[Vergence (optics)|converges]] into a cone within the crystal and is focused to a point on top. The green light is absorbed and spontaneously remitted as red light. Not all of the light is absorbed, and a small portion of the 520&nbsp;nm laser light transmits through the top, unaltered by the ruby's red color.]]
Fluorescence occurs when an excited molecule, atom, or [[nanostructure]], relaxes to a lower energy state (usually the [[ground state]]) through emission of a [[photon]] without a change in [[electron spin]]. When the initial and final states have different multiplicity (spin), the phenomenon is termed [[phosphorescence]].<ref>{{Cite journal |last=Verhoeven |first=J. W. |date=1996-01-01 |title=Glossary of terms used in photochemistry (IUPAC Recommendations 1996) |url=https://www.degruyter.com/document/doi/10.1351/pac199668122223/html |journal=Pure and Applied Chemistry |language=de |volume=68 |issue=12 |pages=2223–2286 |doi=10.1351/pac199668122223 |issn=1365-3075}}</ref>

When a molecule in its ground state (called S<sub>0</sub>) is photoexcited it may end up in any one of a number of excited states (S<sub>1</sub>, S<sub>2</sub>, S<sub>3</sub>,...). These higher excited states are different vibrational levels, populated in proportion to their overlap with the ground state according to the [[Franck-Condon principle]].<ref name="Berberan-Santos, Mario-2012"/>{{rp|31}} These vibrational excited states typically decay rapidly by to S<sub>1</sub>, followed by radiative transition to the ground state or to vibrational states close to the ground state. This transition is called fluorescence. All of these states are [[singlet state]]s.<ref name="Mallick-2023">{{Cite book |last=Mallick |first=Prabal Kumar |url=https://link.springer.com/10.1007/978-981-99-0791-5 |title=Fundamentals of Molecular Spectroscopy |date=2023 |publisher=Springer Nature Singapore |isbn=978-981-99-0790-8 |location=Singapore |language=en |doi=10.1007/978-981-99-0791-5}}</ref>{{rp|225}}

A different pathway for deexcitation is intersystem crossing from the S<sub>1</sub> to a [[triplet state]]   T<sub>1</sub>. Decay from T<sub>1</sub> to S<sub>0</sub> is typically slower and less intense and is called phosphorescence.<ref name="Mallick-2023"/>{{rp|225}}

Absorption of a photon of energy <math>h \nu_{ex} </math> results in an excited state of the same multiplicity (spin) of the ground state, usually a singlet (S<sub>n</sub> with n > 0). In solution, states with n > 1 relax rapidly to the lowest vibrational level of the first excited state (S<sub>1</sub>) by transferring energy to the solvent molecules through non-radiative processes, including  internal conversion followed by vibrational relaxation, in which the energy is dissipated as [[heat]]. Thus the fluorescence energy is typically less than the photoexcitation energy.<ref name="Berberan-Santos, Mario-2012" />{{rp|38}}

The excited state S<sub>1</sub> can relax by other mechanisms that do not involve the emission of light. These processes, called non-radiative processes, compete with fluorescence emission and decrease its efficiency.<ref name="Berberan-Santos, Mario-2012" /> Examples include [[Internal conversion (chemistry)|internal conversion]], [[intersystem crossing]] to the triplet state, and energy transfer to another molecule. An example of energy transfer is [[Förster resonance energy transfer]]. Relaxation from an excited state can also occur through collisional [[Quenching (fluorescence)|quenching]], a process where a molecule (the quencher) collides with the fluorescent molecule during its excited state lifetime. Molecular [[oxygen]] (O<sub>2</sub>) is an extremely efficient quencher of fluorescence because of its unusual triplet ground state.