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Unformatted text preview: Supplementary Notes for Week 5 Changyi Li February 7, 2010 Fluorescence Depolarization Fluorophore excitation is not a reversible process. This is because internal conversion is a quick process compared to uorescence. Since the initial and nal energy level pairs for absorption and emission are di erent, we have potentially two di erent transition dipoles: one for each process. I will outline some of the theories leading to uorescence depolarization. Why are the emission dipole and the absorption dipole di erent? Franck-Condon Principle Franck-Condon principle states that a transition between two states is more likely to happen if the two vibrational wave functions overlap more signi cantly. Wikipedia has a very good derivation and you can nd it here under the section Quantum mechanical formulation : http://en.wikipedia.org/wiki/Franck%E2%80%93Condon_principle The result of the Franck-Condon Principle is that the transition probability, and as a result, the transition rate, is determined by the orbital selection rule (the selection rule for the dipole-dipole transition that was discussed earlier this term), spin-spin selection rule ( S = 0 ), and the Franck-Condon factor, which is a quantitative value measuring the degree of overlap between the vibrational wave functions....
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