Photodissociation

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(Redirected from Photolysis)

Photodissociation (or photolysis) is a chemical reaction in which a chemical compound is broken down by photons. Photodissociation is not limited to visible light, but to have enough energy to break up a molecule; the photon is likely to be an electromagnetic wave with the energy of visible light or higher, such as ultraviolet light, x-rays and gamma rays. The direct process is defined as the interaction of one photon interacting with one target molecule.

1 Role in Photosynthesis
2 Photolysis in the atmosphere
3 Astrophysics
4 Multiple Photon Dissociation
5 See also

[edit] Role in Photosynthesis

Photolysis is a part of photosynthesis, which occurs in the granum of the chloroplast. In photolysis the light absorbed by the chlorophyll is turned into chemical energy which is used to split water into hydrogen and oxygen. The oxygen is released as a byproduct while the hydrogen binds with the coenzyme NADP to form NADPH₂.

[edit] Photolysis in the atmosphere

Photolysis also occurs in the atmosphere as part of a series of reactions by which primary pollutants such as hydrocarbons and nitrogen oxides react to form secondary pollutants such as peroxyacyl nitrates. See photochemical smog.

The two most important photodissociaton reactions in the troposphere are firstly:

O₃ + hν → O₂ + O¹D λ < 320 nm

which generates an excited oxygen atom which can go on to react with water to give the hydroxyl radical:

O¹D + H₂O → 2OH

The hydroxyl radical is central to atmospheric chemistry as it initiates the oxidation of hydrocarbons in the atmosphere and so acts like a detergent.

Secondly the reaction:

NO₂ + hν → NO + O

is a key reaction in the formation of tropospheric ozone.

The formation of the ozone layer is also caused by photodissociation. Ozone in the earth's stratosphere is created by ultraviolet light striking oxygen molecules containing two oxygen atoms (O₂), splitting them into individual oxygen atoms (atomic oxygen); the atomic oxygen then combines with unbroken O₂ to create ozone, O₃. In addition, photolysis is the process by which CFCs are broken down in the upper atmosphere to form ozone-destroying chlorine free radicals.

[edit] Astrophysics

In astrophysics, photodissociation is one of the major processes through which molecules are broken down (but new molecules are being formed). Because of the vacuum of the interstellar medium, molecules and free radicals can exist for a long time. Photodissociation is the main path by which molecules are broken down. Photodissociation rates are very important in the study of the composition of interstellar clouds in which stars are formed.

Typical examples of photodissociation in the interstellar medium are (<math>h\nu</math> is the scientific notation for light, specifically a photon):

<math>H_2O + h\nu \rightarrow H + OH</math>

<math>CH_4 +h\nu \rightarrow CH_3 + H</math>

[edit] Multiple Photon Dissociation

In comparison to ultraviolet or other high energy photons, single photons in the infrared spectral range usually are not energetic enough for direct photodissociation of molecules. However, after absorption of multiple infrared photons a molecule may gain internal energy to overcome its barrier for dissociation. Multiple Photon Dissociation (MPD) can be achieved by applying high power lasers, e.g. a Carbon dioxide laser, or a Free electron laser, or by long interaction times of the molecule with the radiation field without the possibility for rapid cooling, e.g. by collisions. The latter method allows even for MPD induced by black body radiation.