When a molecule absorbs photons, its electrons are excited to higher electronic and vibrational energy levels.

The excited electrons exist in a singlet state relative to the ground-state electron or in a triplet state.

Transition to the ground state involves no spin flips and occurs through a combination of photon-emitting and non-photon-emitting, radiationless processes.

Deactivation pathways are visualized in a Jablonski diagram where S₀ is the ground singlet electronic state.

The three excited electronic states are S₁ and S₂, the first and second electronic singlets, and T₁, the first electronic triplet state.

In radiationless deactivation via vibrational relaxation, the molecule loses energy while moving to a lower vibrational level within the same electronic state.

It causes a shift in the fluorescence band toward lower frequencies or longer wavelengths than the absorption band, causing Stokes shift.

In internal conversion, a molecule moves from an excited electronic state to a lower energy state with the same spin.

External conversion transfers excess energy to the surrounding solvent or sample matrix.

Conversely, intersystem crossing involves transitions between singlet and triplet states.