Fluorescence and Phosphorescence
As requested, here is my explanation of Fluorescence and Phosphorescence. :)
Fluorescence and Phosphorescence are two atomic emission processes which produce light.
Fluorescence:
Safety glasses with strong fluorescence under UV light. Source: (Erty’s Laboratory)
When particular types of molecules are struck with energetic photons (typically in the visible to ultraviolet range), their electrons can become excited. The electrons will move from a ground state (stable orbit) to an excited state (higher unstable orbit). As is with most things in this world, nature desires equilibrium. These excited electrons want to return to their natural, stable orbit. They lose their energy over time naturally by their increased vibrations, but some molecules are capable of producing light (photons) to dissipate this excess energy. This is fluorescence.
Fluorescence occurs when the photon emission is between electrons of the same spin state (i.e. goes from the excited state back to the ground state). Fluorescence is typically a short lived process (from 10^-5 to 10^-8s) compared to phosphorescence (which can last from 10^-4 s to hours). Fluorescence is shorter lived but brighter, and when removed from the source will stop ‘glowing’, where as phosphorescence will continue to glow for a while.
Examples of molecules that fluoresce:
-Fluorene
-Quinine (anti-malarial drug)
Unfortunately very few molecules will naturally fluoresce, so when fluorescence is desired fluorescent molecules (fluorophores) can be introduced into the molecule of interest. Fluorescence is used extensively in analytical chemistry, as the light omitted can be quantified with mathematical relationships to reveal the concentration. Fluorescence is also used often in the biological and medical fields to visualize cells and transport of various chemicals in a body.
Phosphorescence:
An example of phosphorescence. Europium doped strontium silicate-aluminate oxide powder (cyan pigmented). Shown under visible light, long-wave UV light, and in total darkness (source wikipedia)
The process of Phosphorescence is very similar to Fluorescence but differs in the time that the energy is released (as mentioned above). In Phosphorescence, electrons are brought to an excited state by energetic photons absorbed by the molecules. In this case, the excited state is called a triplet state. In these triplet states, in particular molecules, they dissipate energy much slower than with fluorescence. The electrons are almost trapped in this higher state, emitting light constantly. I don’t know more than that. To understand it more you’ll need to know quantum physics!
In contrast to fluorescence, phosphorescence compounds are much more common.
A common real world example of phosphorescence are those glow in the dark stars every kid had on their ceiling. During the day the light from the sun or from lamps excite the electrons in the compounds, from this point on they’re constantly emitting low levels of light. Its only evident when the lights are turned off.
Examples of molecules that phosphoresce (via wikipedia):
Calcium sulfide with strontium sulfide with bismuth as activator, (Ca,Sr)S:Bi, yields blue light with glow times up to 12 hours, red and orange are modifications of the zinc sulfide formula. Red color can be obtained from strontium sulfide.
Zinc sulfide with about 5 ppm of a copper activator is the most common phosphor for the glow-in-the-dark toys and items. It is also called GS phosphor.
Mix of zinc sulfide and cadmium sulfide emit color depending on their ratio; increasing of the CdS content shifts the output color towards longer wavelengths; its persistence ranges between 1–10 hours.
Sources:
My notes
http://www.newton.dep.anl.gov/askasci/chem03/chem03595.htm
http://en.wikipedia.org/wiki/Phosphor#Applications
