Interconversion mechanism, electron-atom

Spin-orbit coupling, which promotes rapid singlet-triplet interconversion, increases with atomic number to the fourth power. Heavy atoms promote this intersystem crossing, both internally as a substituent on a dye molecule, and externally. If Q contains a heavy atom (Hg, I, etc.) quenching of A might be due to stimulated singlet-triplet interconversion. In many of these systems, electron transfer can also occur and this, frequently, is the dominant interaction mechanism. 

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