The Mysterious Factor of Two

On the other hand, Forster showed a lot of insight by including the factor n in the denominator. This factor is not included in our derivation. It depends on the polarizability of the medium, since n is the refractive index. In our derivation, we have not accounted for the fact that there are a number of other electrous present that are not excited. In electrodynamics theory, these electrons are approximately taken into account by the factor of n 2 in the denominator. 

In summary, Forster included the factor of two in the denominator, but left it out in the numerator. The present author included the factor of two in the numerator, but left it out in the denominator. Our final result is thus a factor of four larger than Forster s result. The correct result is the geometrical average. This value is used by the majority of scientists, since they tend to forget about both of the important factors of two. 

In the triplet case, the large term (ai bj) is missing in Equation 14.11. In the term (ab ji), the function ab decreases as exp(-4eR), where e is the orbital energy measured in H. Assuming that e is 0.25 H, the decrease factor of ab is exp(-R). The same is the case with the function ji. The integral (ai bj) thus decreases as exp(-2R). The transition probability for a Dexter transition is proportional to the square of (ai bj) and therefore decreases as exp(-4R). This means that triplet-triplet excitation transfer only takes place at a small distance between the chro-mophores. In practice, one talks about sensitation, meaning a collision between the molecules. 

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