Jablonski diagram
= Jablonski diagram
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A <Jablonski diagram> arranges molecular <energy eigenstates> vertically and draws arrows for changes between them. In the usual closed-shell molecular example, $S_0$ is the electronic <ground state>, $S_1$ and $S_2$ are electronic <excited states> with <singlet state> spin, and $T_1$ has <triplet state> spin. Each electronic state also has levels of <molecular vibration>. The diagram records possible processes, not molecular paths in space.<Advanced Fluorescence Microscopy Techniques—FRAP, FLIP, FLAP, FRET and FLIM by Hellen C. Ishikawa-Ankerhold and others>[§1.1.1, Figure 2B; §1.1.2, Figure 4: singlet and triplet levels]{ref}
<Photon absorption> can populate an upper level of <molecular vibration> in $S_1$. Relaxation removes excess <energy> without <photon emission>; <fluorescence> commonly starts from relaxed $S_1$.<What is the Stokes Shift? by Edinburgh Instruments>[Stokes Shift in Fluorescence Spectroscopy, Figure 4 and its discussion]{ref} The drawing also includes relaxation from a higher electronic <excited state> and a competing spin-changing route to a <triplet state>.
\Image[/fluorescence-jablonski-level-processes.png]
{title=Molecular excitation, relaxation and <fluorescence> in a <Jablonski diagram>}
{id=fluorescence-jablonski-level-processes}
{description=Original schematic for a usual molecular sequence. Straight coloured arrows represent <photon absorption> and <photon emission>; dashed arrows show routes without <photon emission>. Vertical position indicates <energy>, while horizontal spacing only separates processes. The level spacings and arrows are illustrative, not measurements or a prediction of transition rates. The selected emission ends above the lowest $S_0$ level, so subsequent relaxation releases additional <energy> without another fluorescent <photon>.}