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Photochemistry

Luminescence

Although it is not strictly a photochemical reaction, another pathway by which the excited species may reduce its energy is by emitting a photon of light. This process is called luminescence (path viii). Luminescence includes the processes of fluorescence (prompt emission of a photon) and phosphorescence (delayed emission of a photon). Optical brighteners in laundry detergents contain substances that absorb light of one wavelength, usually in the ultraviolet range, but emit light at a longer wavelength, usually in the visible range—thereby appearing to reflect extra visible light and making clothing appear whiter. This process is called fluorescence and only occurs while the substance is being illuminated. The related process, phosphorescence, persists after the excitation source has been removed and is used in "glow-in-the-dark" items.

Resources

Books

Buchanan, B.B., W. Gruissem, and R L. Jones. Biochemistry and Molecular Biology of Plants. Rockville, MD: American Society of Plant Physiologists, 2000.

Lide, D.R., ed. CRC Handbook of Chemistry and Physics Boca Raton: CRC Press, 2001.

Williamson, Samuel J. and Herman Z. Cummins. Light and Color in Nature and Art. New York: John Wiley and Sons, 1983.

Periodicals

Li, X. P., Bjorkman, O., Shih, C., et al. "A Pigment-binding Protein Essential for Regulation of Photosynthetic Light Harvesting." Nature 403 (January 2000): 391-395.

Toon, Owen B., and Richard P. Turco. "Polar Stratospheric Clouds and Ozone Depletion." Scientific American no. 264 (1991): 68-74.

Wayne, Richard. Principles and Applications of Photochemistry Oxford: Oxford Science Publications, 1988.

Zewail, Ahmed. "The Birth of Molecules." Scientific American no. 263 (1990): 76-82.

Karen Trentelman

KEY TERMS

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Absorption: —The acquisition of energy from a photon of light by an atomic or molecular species, often causing electronic excitation.
Electronic excitation: —The state of an atom or molecule in which an electron has been given additional energy.
Emission: —The generation of a photon of light from an electronically excited atomic or molecular species in order to reduce its total energy.
Photodissociation: —The breaking of one or more molecular bonds resulting from the absorption of light energy.
Photon: —A quantum, or discrete packet, of light energy.
Quantum yield: —In a photochemical reaction, the number of product species divided by the number of photons that were absorbed by the reactant.
Wavelength: —The distance between two consecutive crests or troughs in a wave.

Additional topics

Science EncyclopediaScience & Philosophy: Philosophy of Mind - Early Ideas to Planck lengthPhotochemistry - The Basic Laws Of Photochemistry, Photochemistry Induced By Visible And Ultraviolet Light, Reaction Pathways, Dissociation - Ionization, Isomerization