PHOTOCHEMICAL REACTION

any type of chemical process initiated by the absorption of energy in the form of visible, infrared, or ultraviolet radiation. The immediate consequence of this absorption is called the primary photochemical process. Subsequent changes, called secondary processes, are part of photochemistry. The process by which a photochemical reaction is carried on is photolysis. Photochemistry differs from most other aspects of chemistry in one regard. If an atom or molecule absorbs energy from a beam of light, it gains far more energy than it ever could by other methods; e.g., from ordinary heating. Consequently, photochemical processes are sometimes extremely efficient for the conversion of energy from light into chemical energy. The most important single photochemical process for living systems, photosynthesis, is of this type: green plants convert light energy into stored chemical energy by producing carbohydrates from carbon dioxide and water. Many other natural processes are wholly or partly photochemical. Ozone in the upper atmosphere, which shuts out most of the Sun's intense ultraviolet radiation, is produced by the action of light on oxygen molecules: sunlight breaks some of the molecules into atoms, which combine with other oxygen molecules to make ozone. Bleaching laundry in sunlight is at least partly a photochemical process, and so is the darkening of white lithopone (zinc sulfide) paint. Photography is based on a photochemical process, the action of light on grains of silver chloride or silver bromide. any type of chemical process initiated by the absorption of visible, ultraviolet, or infrared radiation. The most important photochemical process for living systems is photosynthesis, the production by green plants of carbohydrates from carbon dioxide and water. Vision in animals depends on photochemical reactions that occur in the eye, and reactions of this kind are involved in photography, bleaching of laundry and tanning of the skin by sunlight, and in numerous processes used in the chemical industry. For a photochemical process to occur, energy in the form of visible, infrared, or ultraviolet radiation must be absorbed by a substance. Primary photochemical processes are those that occur as an immediate consequence of this absorption. Secondary processes are those subsequent changes that follow the primary processes. The core of photochemical theory is derived mainly from the principles of quantum mechanics and of atomic and molecular physics. Photochemical reactions often convert light energy into chemical energy very efficiently. This is a consequence of the fact that when an atom or molecule absorbs light energy it gains much more energy than it ever could by other methods such as heating. An example of this type of photochemical process is photosynthesis. Another important photochemically based natural phenomenon is the production of ozone in the upper atmosphere. Ozone, which filters out most of the harmful ultraviolet radiation from the Sun, is produced when sunlight breaks the bonds of some oxygen molecules, allowing atomic oxygen to combine with oxygen molecules to yield ozone. Photography employs another type of photochemical process. Grains of silver chloride or silver bromide in photographic film absorb minute amounts of light when the camera shutter opens, thereby converting bound silver to metallic silver. Conventional chemical action (development of the film) then amplifies the photochemical action of light alone. Additional reading Victor Henri, tudes de photochimie (1919), is important for the history of the subject. Jack G. Calvert and James N. Pitts, Jr., Photochemistry (1966), is a widely used monograph on the subject, covering basic generalities and specific applications. William Albert Noyes, Jr., and Philip Albert Leighton, The Photochemistry of Gases (1941, reissued 1966), a classic, is probably an easier introduction to the subject. More recent works include Ralph Roberts, Applications of Photochemistry (1984); Richard P. Wayne, Principles and Applications of Photochemistry (1988); C.H.J. Wells, Introduction to Molecular Photochemistry (1972); J.D. Coyle, Introduction to Organic Photochemistry (1986); and Nicholas J. Turro, Modern Molecular Photochemistry (1978, reissued 1991). Advances in Photochemistry (irregular), surveys current work in specific areas. R.M. Noyes and A. Weller, "Photostationary Methods," and G. Porter, "Flash Photolysis," both in Technique of Organic Chemistry, vol. 8, pt. 2 (1963), present clear discussions of most of the experimental methods, except lasers; these are treated in David L. Andrews, Lasers in Chemistry, 2nd ed. (1990). R. Stephen Berry The Editors of the Encyclopdia Britannica