Discover

'Internal conversion' is defined as a transition between one set of atomic (or molecular) electronic excited levels to another set of the same spin multiplicity (for example, the second singlet state to the first singlet state). It is sometimes called "radiationless de-excitation", because no photons are emitted. It differs from intersystem crossing in that, while both are radiationless methods of de-excitation, the molecular spin state for internal conversion remains the same, whereas it changes for intersystem crossing.
A classic example of this process is the quinine sulphate fluorescence, which can be quenched by the use of various halide salts. What happens is that the excited molecule can de-excite by increasing the thermal energy of the surrounding solvated ions.
A general and quantitative discussion of intramolecular radiationless transitions is the subject of an article by M. Bixon and J. Jortner (''J. Chem. Phys.'', '48' (2) 715-726 (1986)).