= Michael Kasha
{c}
\b[Michael Kasha] connected molecular light emission with the structure and competing relaxation of <excited states>. His 1950 paper formulated the spectroscopic regularity now called <Kasha's rule>: emission of a given <spin multiplicity> normally comes from the lowest excited level of that multiplicity.<Characterization of electronic transitions in complex molecules by Michael Kasha>[Printed pp.14–15, Abstract and Internal Conversion]{ref}
The argument involves rates, rather than a prohibition on preparing a higher <excited state>. In an original model, a higher state has radiative rate $k_r$ and competing <molecular internal conversion> rate $k_{ic}$. Its probability of emitting before conversion is
$$
P_{\mathrm{higher\ emission}}=\frac{k_r}{k_r+k_{ic}}.
$$
Choosing $k_r=10^8\,\mathrm{s^{-1}}$ and $k_{ic}=10^{12}\,\mathrm{s^{-1}}$ gives approximately $10^{-4}$. Most molecules can therefore reach the lower state before emitting even when absorption initially populates the higher state. These are illustrative rates, not measured constants assigned to the 1950 paper. A rate competition also explains why the regularity has conditions and exceptions.
Kasha later coauthored the 1979 investigation with <Pradeep K. Sengupta> of <3-Hydroxyflavone> and <quercetin> <fluorescence>. Comparing solution spectra, rigid low-temperature matrices, excitation spectra and isotope substitution supported formation of an emitting <tautomer> through excited-state <proton transfer>.<Excited state proton-transfer spectroscopy of 3-hydroxyflavone and quercetin by Pradeep K. Sengupta and Michael Kasha>[Printed pp.382–384, Figures1–4 and discussion]{ref} This distinguishes the electronic state initially reached by absorption from the molecular form that eventually emits. The lesson is experimental: emission colour and lifetime can identify competing pathways that a molecular formula alone cannot reveal.
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