 Molecular internal conversion

Molecular internal conversion is a radiationless transition between electronic states of the same spin multiplicity. Electronic and vibrational energy are redistributed without emission of a photon in the transition itself. IUPAC describes the step as isoenergetic: a lower electronic state can be reached with increased vibrational excitation, followed by subsequent relaxation.^[ref]

This is distinct from intersystem crossing, which connects states of different multiplicity, and from vibrational relaxation within an electronic state. It is also distinct from nuclear internal conversion. A molecular singlet-to-singlet transition such as $S_2\to S_1$ fits the molecular definition; the energy need not be immediately transferred as heat to a surrounding solvent.

An original population model separates internal conversion from later decay. Suppose an impulsive excitation places population $N_0$ in $S_2$, transfer to $S_1$ occurs at rate $k_{21}$, and $S_1$ loses population at total rate $k_1$:For $k_{21}\ne k_1$,The lower-state population first rises and later decays. If only part of $k_1$ is radiative, fluorescence intensity is proportional to that radiative rate times $N_1$, not simply to the loss rate of $S_2$.

For stipulated $k_{21}=10^{12}{\mathrm{s}}^{-1}$ and $k_1=10^8{\mathrm{s}}^{-1}$, the transfer time scale is 1 ps while the lower-state lifetime is 10 ns. A fast population transfer can therefore precede much slower emission. These are model constants, not measured values assigned to every molecule.

An informative experiment resolves the time dependence of upper- and lower-state signals after a pulse and tests whether the proposed transfer accounts for both. A decay trace alone can have several explanations; spectral assignment and competing channels matter. Sequential excited-state population kinetics is one model to test against those measurements.

Michael Kasha's 1950 analysis linked radiationless transitions among like-multiplicity states to emission from the lowest excited level of a given multiplicity.^[ref] The rate competition supplies a physical motivation for Kasha's rule, while leaving room for molecules whose competing pathways behave differently.