The primary process in photochemistry is the absorption of light by a molecule, which leads to the excitation of an electron to a higher energy level.

The excited state of a molecule is referred to as a singlet state if the spins of the two excited electrons are paired, and a triplet state if the spins are unpaired.

Einstein's Photoelectric Effect states that the energy of an absorbed photon is equal to the work function of the material plus the kinetic energy of the emitted electron.

Fluorescence is the process by which an excited molecule returns to its ground state, releasing energy in the form of light with the same wavelength as the absorbed light.

Förster Resonance Energy Transfer (FRET) is the process by which an excited molecule transfers its energy to another molecule through non-radiative dipole-dipole interactions.

Photolysis is the term for the chemical reaction initiated by the absorption of light, which results in the breaking of chemical bonds.

Photoisomerization is the term for the photochemical reaction in which a molecule absorbs a photon and undergoes a change in its molecular structure, resulting in the formation of an isomer.

Photocycloaddition is the term for the photochemical reaction in which two molecules combine to form a single molecule, typically involving a [2+2] or [4+2] cycloaddition reaction.

Photooxidation is the term for the photochemical reaction in which a molecule undergoes oxidation, typically involving the transfer of electrons to an oxidizing agent.

Photoreduction is the term for the photochemical reaction in which a molecule undergoes reduction, typically involving the transfer of electrons from a reducing agent.

Photoionization is the term for the photochemical reaction in which a molecule loses an electron, typically resulting in the formation of a positive ion.

Photodissociation is the term for the photochemical reaction in which a molecule breaks apart into smaller fragments, typically involving the breaking of chemical bonds.

Intersystem Crossing is the term for the photochemical reaction in which a molecule undergoes a change in its spin state, typically from a singlet state to a triplet state or vice versa.

Internal Conversion is the term for the photochemical reaction in which a molecule undergoes a change in its electronic configuration, typically from a higher energy state to a lower energy state, without emitting light.

Fluorescence is the term for the photochemical reaction in which a molecule emits light after absorbing energy, typically with a short delay after excitation.