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Photochemistry

Photochemistry

Photochemistry is the study of chemical changes made possible by light energy. The production of ozone in Earth's upper atmosphere is an example of such a change. Light from the Sun (solar energy) strikes oxygen molecules in the stratosphere, causing them to break down into two oxygen atoms:

O2 + hν O + O

(The expression hν is commonly used to represent a unit of light energy known as the photon.)

In the next stage of that reaction, oxygen atoms react with oxygen molecules to produce ozone (O3):

O + O2 O3

Steps in photochemical processes

The excited state. A photochemical change takes place in two steps. Imagine that a light beam is shined on a piece of gold. The light beam can be thought of as a stream of photons, tiny packages of energy. The energy of the photon is expressed by means of the unit hν.

When a photon strikes an atom of gold, it may be absorbed by an electron in the gold atom. The electron then becomes excited, meaning that it has more energy than it did before being hit by the photon. Chemists use an asterisk (*) to indicated that something is in an excited state. Thus, the collision of a photon with an electron (e) can be represented as follows:

e + hν e*

Once an electron is excited, the whole atom in which it resides is also excited. Another way to represent the same change, then, is to show that the gold atom (Au) becomes excited when struck by a photon:

Au + hν Au*

Emission of energy. Electrons, atoms, and molecules normally do not remain in an excited state for very long. They tend to give off their excess energy very quickly and return to their original state. When they do so, they often undergo a chemical change. Since this change was originally made possible by absorbed light energy, it is known as a photochemical change.

The formation of ozone is just one example of the many kinds of photochemical changes that can occur. When solar energy breaks an oxygen molecule into two parts, one or both of the oxygen atoms formed may be excited. Another way to write the very first equation above is as follows:

O2 + hν O* + O

The excited oxygen atom (O*) then has the excess energy needed to react with a second oxygen molecule to form ozone:

O* + O2 O3

Another way for an excited atom or molecule to lose its energy is to give it off as light. This process is just the reverse of the process by which the atom or molecule first became excited. If the atom or molecule gives off its excess energy almost immediately, the material in which it is contained glows very briefly, a process known as fluorescence. If the excess energy is given off more slowly over a period of time, the process is known as phosphorescence. Both fluorescence and phosphorescence are examples of the general process of light emission by excited materials known as luminescence.

[See also Atom; Luminescence; Photosynthesis ]

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photochemistry

photochemistry, study of chemical processes that are accompanied by or catalyzed by the emission or absorption of visible light or ultraviolet radiation. A molecule in its ground (unexcited) state can absorb a quantum of light energy, or photon, and go to a higher-energy state, or excited state (see quantum theory). Such a molecule is then much more reactive than a ground-state molecule and can undergo entirely different reactions than the more stable molecule, following several different reaction pathways. One possibility is that it can simply emit the absorbed light and fall back to the ground state. This process, called chemiluminescence, is illustrated by various glow-in-the-dark objects. Another possibility is for the molecule to take part in a photo-induced chemical reaction; it may break apart (photodissociate), rearrange, isomerize, dimerize, eliminate or add small molecules, or even transfer its energy to another molecule. Photochromic compounds—compounds that change color reversibly in going from the dark to the light—are generally compounds that are capable of reversible isomerization, or rearrangement. In the absence of light, the compound exists in its most stable form, which exhibits a particular color; in the presence of light, the compound goes to a less stable form, which exhibits a different color. After removal of the light, the compound will revert back to its original state. The best-known and most important photochemical reaction is photosynthesis, the complex, chlorophyll-catalyzed synthesis of sugars from carbon dioxide and water in the presence of light. Other extremely important and complex photochemical reactions take place in the eye. Photochemistry is indispensible to industries involved with dyes, photography, television, and many other applications of light and color.

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photochemistry

pho·to·chem·is·try / ˌfōtōˈkeməstrē/ • n. the branch of chemistry concerned with the chemical effects of light.

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photochemistry

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