The excited states of jewelry

The concept of fluorescence comes from fluorite: a mineral consisting of the salt calcium fluoride (CaF2), a known fluorescent substance.

Types of fluorescence
Fluorescence in the region of ultraviolet radiation and visible light is caused by excited states of an electron in an atom: the energy differences between the paths of the binding electrons there are in the order of a few electron volts (eV). A small change in the atom will change the energy levels a little, and therefore also the wavelength at which the fluorescence occurs. These effects can be used to identify the molecules.

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Fluorescence (X-ray fluorescence) can also occur for X-rays, but these are the inner, most tightly bound electrons of heavier atoms. The energy differences there are of the order of a few to tens of keV. These energy levels do not change measurably when the atom is incorporated into a molecule, so these fluorescence phenomena can be used to determine the types of atoms that make up a sample independently of the chemical bonds. This technique is called X-ray fluorescence spectrometry.

Application
There are both naturally occurring and synthesized materials with fluorescent properties. The best-known examples are the fluorescent tube and the low-energy light bulb. Here ultraviolet radiation is generated by an electrical discharge in a diluted gas in a tube. This UV light will then be converted into visible light by a powder applied to the inside of the tube. Such powders are often called phosphors although they do not contain phosphorus.

The same principle is used when checking the authenticity of banknotes. In this case, the banknotes are equipped with fluorescent substances, which can be made visible by means of a UV lamp.

Furthermore, fluorescent colors are used in clothing that should stand out in traffic (the orange vests of road workers), and in yellow, green and orange pens to mark text. Because such colours convert short wavelengths of blue light into colours with longer wavelengths, under some lighting conditions (disco, UV light) they do indeed appear to emit light themselves. (Under a UV lamp, also known as a black light, teeth sometimes appear green.) Even optical brighteners in detergents convert UV light into blue visible light by fluorescence.

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During some chemical reactions, luminescence can also occur. A well-known example is an oxidation of luminol, in which an oxygen atom is temporarily put into an excited state. The reaction of luminol is not fluorescence but chemoluminescence.

In fluorescence microscopy, fluorescent proteins (e.g. Green Fluorescent Protein (GFP)) are used in, among other things, DNA research. This technique is called FISH: fluorescence-in-situ hybridization. Fluorescent proteins occur naturally in some species of jellyfish (Aequorea victoria) and coral (Discosoma). Thanks to genetic modification it is possible to give fluorescent properties to other animals as well. The best known example of this is the night pearl (fluorescent aquarium fish) which has been sold in Taiwan since 2003.

Fluorescence must be distinguished from phosphorescence, the phenomenon that a substance remains after being exposed in the dark for some time. This is used, among other things, in paint on the hands of clocks.

Phosphorescence is a special case of luminescence. It is the phenomenon that a fabric continues to glow in the dark for some time after being exposed. Phosphorescence can also occur when such a material is bombarded with accelerated electrons, such as in a cathode ray tube.

The term means approximately lights like phosphorus. White phosphorus does indeed give light in the dark, but with this substance this is caused by oxidation reactions of the phosphorus with oxygen from the air (it can also ignite spontaneously), and so the light has a different origin. In the usual naluminous substances, it is a result of the slow fall back of electrons excited by irradiation with light. The fact that this happens slowly is because the return of the electrons to the ground state in quantum mechanics is a forbidden transition.

While studying phosphorescence, Henri Becquerel discovered the phenomenon of radio-activity in 1896.

Phosphorescent paint was used in the past[source?] on, among other things, the hands and dials of watches so that they could still be read in the dark for a few hours. It is also used in light switches, so that they can be found in the dark, and in toys for a children’s room.