photochemistry

موضوع: photochemistry الخميس 27 نوفمبر 2008, 12:35 am

Photochemistry :-
is the study of the interactions between atoms, small molecules, and light (or electromagnetic radiation).Photochemistry may also be introduced to laymen as a reaction that proceeds with the absorption of light. Normally a reaction (not just a photochemical reaction) occurs when a molecule gains the necessary activation energy to undergo change.The absorption of a photon of light by a reactant molecule may also permit a reaction to occur not just by bringing the molecule to the necessary activation energy, but also by changing the symmetry of the molecule's electronic configuration, enabling an otherwise inaccessible reaction path
Applications:-
There are important processes based in the photochemistry principles. One case is photosynthesis, which some plants use light to create glucose in their chloroplasts to contribute to cell metabolism.

Quantum yield:-

The quantum yield of a radiation-induced process is the number of times that a defined event occurs per photon absorbed by the system. Thus, the quantum yield is a measure of the efficiency with which absorbed light produces some effect.

The quantum efficiency depends on both the absorption of light and the collection of charges. Once a photon has been absorbed and has generated an electron-hole pair, these charges must be separated and collected at the junction.

Photochemistry is concerned with the absorption, excitation and emission of photons by atoms, atomic ions, molecules, molecular ions, etc. The simplest photochemical process is seen with the absorption and subsequent emission of a photon by a gas phase atom such as sodium.

When the sodium atom absorbs a photon it is said to be excited. After a short period of time, the excited state sodium atom emits a photon of 589 nm light and falls back to the ground state:

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The atom can be excited by a flame, and this is the basis of the well known "flame test" of Group 1 and 2 metal salts (LiCl, NaBr, CaCl2, SrCl2, etc.) and atomic emission spectroscopy.:

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Or the sodium atoms can be electrically excited and this is the basis of energy efficient orange sodium street lights:

Photochemical Reaction Processes

A photoexcited species, [X-Y]*, can relax (react) via a variety of pathways.

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موضوع: رد: photochemistry الخميس 27 نوفمبر 2008, 12:42 am

Photochemical Reactions

A photochemical reaction occurs when internal conversion and relaxation of an excited state leads to a ground state isomer of the initial substrate molecule, or when an excited state undergoes an intermolecular addition to another reactant molecule in the ground state. The cis-trans photochemical isomerization of stilbene is a reaction of the first kind, as shown in the following diagram. Both cis and trans-stilbene undergo π → π* electron excitation by absorption of uv light.

موضوع: رد: photochemistry الخميس 27 نوفمبر 2008, 12:42 am

A sensitiser in chemoluminescence is a chemical compound, capable of light emission after it has received energy from a molecule, which became excited previously in the chemical reaction.

موضوع: رد: photochemistry الخميس 27 نوفمبر 2008, 12:48 am

Handbook of Photochemistry, Third Edition
(Hardcover)
by Marco Montalti (Author), Alberto Credi (Author), Luca Prodi (Author), M. Teresa Gandolfi

Product Details

Hardcover: 664 pages
Publisher: CRC; 3 edition
(February 21, 2006)
Language: English
ISBN-10: 0824723775
ISBN-13: 978-0824723774
Product Dimensions: 9.3 x 6.2 x 1.4 inches
Shipping Weight: 2.3 pounds

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موضوع: رد: photochemistry الخميس 27 نوفمبر 2008, 1:33 am

يا سلام عليك ربنا يكرنك بفضله

موضوع: رد: photochemistry الإثنين 05 يناير 2009, 3:37 am

بس أيضاح مطلوب بعد الشئ لبعض التفاصيل علشان الاستفادة بس أحب أقولك برضوا جزاك الله خيرا