What is flourescence? How does it work?
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
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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Update: June 2012