What is fluorescence and how does it arise?

[retupmoc]

What is fluorescence and how does it arise?

Is it just a spontneous de-excitation after the electron in the molecule (protein) has been excited. This is a 6 mark question in last years Chemistry-2 exam and i don't really know how to go about answering it. Any suggestions?

 

[HallsofIvy]

At http://www.Newton.dep.anl.gov/askasci/phy99/phy99540.htm
it says:

Question - 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 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.

 

[retupmoc]

thanks for the help