= Kasha-Vavilov rule
{c}
The \b[Kasha-Vavilov rule] states that the <quantum yield> of luminescence is normally independent of excitation wavelength. IUPAC explicitly notes exceptions.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book)>[Kasha–Vavilov rule K03371]{ref} The statement concerns emitted photons per absorbed photon, not equal detector counts under equal incident optical power.
An original photon-accounting model writes
$$
\Phi=\frac{N_{\rm emitted}}{N_{\rm absorbed}},\qquad
\dot N_{\rm absorbed}=\frac{P\lambda}{hc}(1-10^{-A}),
$$
for monochromatic incident power $P$, wavelength $\lambda$ and absorbance $A$. Comparing raw fluorescence at different wavelengths without correcting incident photon number and absorbed fraction can therefore give a misleading yield comparison.
For equal incident powers and equal absorbed fractions at 400 and 500 nm, the absorbed photon rates have ratio $500/400=1.25$. A molecule obeying the rule with unchanged yield then emits 25% more photons per second under the 500 nm condition. Unequal raw emission is entirely consistent with equal emitted-per-absorbed-photon yield.
<Absorbed-photon normalization in quantum-yield comparisons> also needs detector-response correction. The observation system measures a selected fraction of an emission spectrum, not automatically all emitted photons. Keep excitation and emission spectral responses, sample geometry and optical density in the comparison.
The distinction from <Kasha's rule> is physical as well as verbal. Kasha's rule concerns the emitting electronic state. Suppose a wavelength-dependent fraction $f(\lambda)$ of absorbed excitations reaches the same emitting state, whose radiative and nonradiative rates are $k_r,k_{nr}$. Then
$$
\Phi(\lambda)=f(\lambda)\frac{k_r}{k_r+k_{nr}}.
$$
The same emitting state and band shape can coexist with a wavelength-dependent yield if upstream branching changes. Fast <molecular internal conversion> can make the simple wavelength-independent approximation useful, but does not prove it for every system.
A proposed experiment compares corrected integrated emission with absorbed photon counts while checking concentration, chemical stability and competing absorption. A persistent excitation-dependent yield after those corrections is a scientific result to explain, rather than something to normalize away by assuming the rule in advance.
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