User:Nadiezda Fernandez-Oropeza/Notebook/Notebook/2010/11/11
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Fluorescence
DefinitionFluorescence is the emission of electromagnetic radiation, especially of visible light, stimulated in a substance by the absorption of incident radiation and persisting only as long as the stimulating radiation is continued. Cycling of FluorescenceSome molecules are capable of being excited via absorption of light energy to a higher energy state, an excited state. The energy of the excited state decays resulting in the emission of light energy. This process is called Fluorescence. A fluorophore is a molecule capable of fluorescencing. In its ground state the fluorophore molecule is in a relatively low energy configuration, and it does not fluoresce.
When light from an external source hits a fluorophore molecule, the molecule can absorb the light energy. If the energy absorbed is sufficient, the molecule reaches an exited state. This process is known as excitation. [math]\displaystyle{ S_{0}+hv_{ex}=S_{1} }[/math] There are multiple excited states or energy levels that the flourophore can attain depending on the wavelength and energy of the external light source.
Since the fluorophore is unstable at high energy configurations, it eventually adopts the lowest energy excited state, which is semi-stable. In this process there is a loss of energy. The length of time that the fluorophore is in an excited state is called the excited lifetime. It last for a very short time, ranging from 10-15 to 10-9 seconds.
Next, the flourophore rearranges form the semi-stable excited state back to the ground state and the excess energy is released and emitted as light. S_1→S_0+hv_em+heat The emitted light is of lower energy and has a longer wavelength than the absorbed light. This means that the color of the light that is emitted is different from the color of the light that has been absorbed. PhotobleachingAn important characteristic of fluorophores is that they can go through the cycle of fluorescence repeatedly, in theory, indefinitely. However, in reality the fluorophore instability during the excited lifetime makes it susceptible to degradation. High intensity illumination can cause the fluorophore to change its structure and no longer fluoresce. This phenomenon is known as photobleaching.
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