Fig. 3.

Jablonski diagrams showing the main energy pathways for different modes of fluorescence (energy diagrams not drawn to scale). A – single photon Stokes process (i) photon absorption gives excited state, (ii) internal conversion to S₁, first singlet excited state, (iii) fluorescence (emission of photon), (iv) nonradiative decay, (v) intersystem crossing to T₁ (‘forbidden’ triplet excited state), (vi) phosphorescence, and (vii) nonradiative decay.²⁴ B – two-photon upconversion (anti-Stokes process). Excited-state absorption (ESA) proceeds via sequential absorption of two photons to give the excited state and a subsequent emission event. In energy transfer upconversion (ETU), an ion directly absorbs one photon while a neighboring ion absorbs another and transfers the energy to the first ion, resulting in upconversion and emission.²² Reproduced from Lavis and Raines²⁴ and Haase and Schafer.²²