 Radiationless transition

A radiationless transition changes a system's electronic state without absorption or emission of a photon in that transition. Energy is redistributed among coupled degrees of freedom; it does not disappear. In molecular photophysics, the electronic change can populate vibrational motion before subsequent energy transfer to the surroundings.^[ref]

Molecular internal conversion connects electronic states of the same spin multiplicity, while intersystem crossing connects states of different multiplicity. These must be distinguished from vibrational relaxation within an electronic state and from nuclear internal conversion. A radiationless step need not instantaneously deliver all the electronic energy as heat to a solvent.

A simple competing-rate experiment illustrates why a dark channel can be inferred from light measurements. Suppose an emitting excited population loses members by fluorescence at rate $k_r$ and by nonfluorescent channels at total rate $k_{nr}$. With no population returning,These are the stated one-population assumptions of the fluorescence quantum yield model. For chosen rates $k_r=10^7{\mathrm{s}}^{-1}$ and $k_{nr}=4\times 10^7{\mathrm{s}}^{-1}$, the lifetime is $20\mathrm{ns}$ and the yield is $0.20$. A shorter lifetime here does not mean a larger radiative rate.

With a measured lifetime and yield, the model gives $k_r={\Phi }_f/\tau$ and $k_{nr}=(1-{\Phi }_f)/\tau$. Measuring only an emission intensity is less specific because it also depends on absorbed excitation and detection efficiency. Additional intermediate states, energy transfer or population return require a more complete model; the inferred missing fluorescence does not uniquely identify one radiationless mechanism.

Time-resolved emission paired with transient absorption can therefore test a proposed pathway: the first observes photons from an emitting state, while the second can reveal population in a state that does not emit in the monitored band. A mechanism must account for both signals and their time dependence.