 Kasha-Vavilov rule

The Kasha-Vavilov rule states that the quantum yield of luminescence is normally independent of excitation wavelength. IUPAC explicitly notes exceptions.^[ref] The statement concerns emitted photons per absorbed photon, not equal detector counts under equal incident optical power.

An original photon-accounting model writesfor 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 $k_r,k_{nr}$. ThenThe 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.