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Name: Bonni
Status: other
Age: 14
Location: N/A
Country: N/A
Date: 1999 


Question:
What is flourescence? How does it work?


Replies:
Fluorescence is when a molecule absorbs light of one energy and later emits light of a lower energy. Briefly, what happens is that upon absorption of the first photon, an electron is promoted to a higher-energy orbital. This raises the energy of the molecule by an amount equal to the energy of the photon absorbed.

Electrons move very fast, as they are much lighter (factors over 1000) than atomic nuclei. When the electron is in its new higher-energy state, the most favorable arrangement of the nuclei changes a little. Because the nuclei are so heavy, they will (comparatively) slowly reorganize to their most favorable arrangement. This lowers the total energy of the molecule. It is still higher than in the ground state, but not as much higher as the energy of the absorbed photon.

Eventually, the promoted electron will drop back down to its ground-state orbital. In doing so, a photon of light is emitted, with an energy equal to the change in energy of the molecule. Recall that the molecule's atomic nuclei are no longer in their ground-state geometry, so its new energy is a bit higher than in the original ground state. So, the energy of the emitted photon will be less than the energy of the absorbed photon because of the energy cost of relaxation of the nuclei, both before and after emission of the second photon.

More briefly, the molecule's ground-state energy is G. The absorbed photon's energy is H1, so the energy of the molecule immediately after excitation is G + H1. The nuclei relax, lowering the energy by R1, so the molecule's energy becomes G + H1 - R1. The excited electron drops back down to its ground-state orbital, emitting a photon with energy H2. The total energy of the molecule is then G + H1 - R1 - H2. After the nuclei relax for the final time, the molecule's energy is further lowered by R2, to a final energy of G, back where it started. So,

G = G + H1 - R1 - H2 - R2 (initial energy equals final energy)

0 = H1 - R1 - H2 - R2

H2 = H1 - R1 - R2

in other words, H2, the energy of the emitted photon, is less than H1, the energy of the absorbed photon, by the amount R1 + R2. This energy difference is manifest as heat.

Richard E. Barrans Jr., Ph.D.



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