To understand this phenomenon you first have to understand the structure of atoms and molecules, particularly with reference to the all important electron.
Atoms are the building blocks of matter, each element is made of particular atoms, unique to that element. Atoms themselves are made up of subatomic particles;
Atoms are of the order 1 - 5 angstrom (1 angstrom = 1 x 10-10 m = 0.0000000001 m) in diameter. The neutrons and protons are in the tiny, dense, nucleus of the atom, which has a diameter of about 1 x 10-4 angstrom. All atoms of the same element have the same number of protons in the nucleus. |
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The electrons reside in a cloud around the nucleus and are responsible for the chemistry of an atom.
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Molecules are collections of atoms held together by chemical bonds. Chemical bonds are represented in chemical structures as a single straight line, they are actually pairs of electrons shared between atoms in a molecule. |
The electrons in a molecule have energy associated with them, this energy can only take particular values, that is to say it is quantised. The energy at which the molecule is most stable is called the ground state, if one or more electrons is raised to a higher energy level the molecule is said to be in an electronically excited state. This can be shown diagrammatically;
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Excitation of a molecule can happen in several ways including absorption of light or as a result of a chemical reaction. Molecules preferentially exist in their lowest energy state, an excited molecule will thus lose energy with the electron falling back down the energy staircase to the ground state. For some molecules the excess energy is lost in the form of light, it is this process that is known as luminescence.
A common every day application of luminescence is washing powder that contains optical brightener. The optical brightener is nothing more than a fluorescent dye that shows a blue luminescence when excited by the ultraviolet radiation present in sunlight. The optical brightener is added to counteract the yellowing of cotton as it ages.
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ordinary illumination |
ultra-violet light |
Photographs courtesy of Prof. Richard Russell, Deakin University |
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Luminescence is used forensically to detect fingerprints, it also found application in de-bunking the fake Hitler diaries.
If the luminescence is the result of the molecule being excited by a chemical reaction, the resulting emission of light is called chemiluminescence. This is used forensically in the luminol test for blood.
