S s n s 1 s 2 energy electronic ground state a ic t 2

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S 0 S n S 1 S 2 Energy electronic ground state A IC T 2 T 1 ISC IC P excited vibrational states (excited rotational states not shown) A = photon absorption F = fluorescence (emission) P = phosphorescence S = singlet state T = triplet state IC = internal conversion ISC = intersystem crossing F
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Three nonradiative deactivation processes are also significant here: internal conversion (IC), intersystem crossing (ISC) and vibrational relaxation. Examples of the first two can be seen in the diagram. Internal conversion is the radiationless transition between energy states of the same spin state (compare with fluorescence- a radiative process). Intersystem crossing is a radiationless transition between different spin states (compare to phosphorescence). Vibrational relaxation, the most common of the three-for most molecules, occurs very quickly (<1 x 10 -12 seconds) and is enhanced by physical contact of an excited molecule with other particles with which energy, in the form of vibrations and rotations, can be transferred through collisions. This means that most excited state molecules never emit any energy because in liquid samples the solvent or, in gas phase samples, other gas phase molecules that are present “steal” the energy before other deactivation pro- cesses can occur. These notes were written by Dr. Thomas G. Chasteen at Sam Houston State Uni- versity, Huntsville, Texas.
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