Photochemical equivalence

Abnormally high quantum yields may occur in photochemical reactions. Einstein s law of photochemical equivalence is the principle that light is absorbed by molecules in discrete amounts as an individual molecular process (i.e., one molecule absorbs one photon at a time). From optical measurements it is possible to determine quantitatively the number of photons absorbed in the course of a reaction and, from analyses of the product mixture, it is possible to determine the number of molecules that have reacted. The quantum yield is defined as the ratio of the number of molecules reacting to the number of photons absorbed. If this quantity exceeds unity, it provides unambiguous evidence for the existence of secondary processes and thus indicates the presence of unstable intermediates. 

Stark-Einstein law of photochemical equivalence one photon of radiation can be absorbed only by one molecule [201,202]. 

In 1912, Einstein extended the concept of quantum theory of radiation to photochemical processes and stated that each quantum of radiation absorbed by molecule activates one molecule in the primary step of a photochemical process . This is known as Einstein law of photochemical equivalence. 

According to law of photochemical equivalence, the energy absorbed per mole E is given by... 

State and explain the law of photochemical equivalence and quantum efficiency . 

Einstein photochemical equivalence law phys chem The law that each molecule taking part in a chemical reaction caused by electromagnetic radiation absorbs one photon of the radiation. Also known as Stark-Elnstein law. Tn.stTn fod o kem-a-kol i kwivo-lons, 16 ... 

In simple instances, therefore, we should expect to find one molecule transformed for each quantum of light absorbed, provided that the light is active at all. This is Einstein s law of photochemical equivalence. 

The law of photochemical equivalence states that one molecule absorbs one quantum, which allows a calculation of the rate of formation of radicals from the intensity of the radiation absorbed. This can be extremely useful in the kinetic analysis as it gives a rate constant for the first step in the sequence. For example... 

Walter Noddack (1893-1960) began studying chemistry, physics and mathematics at the University of Berlin in 1912. Having volunteered during World War I, he received his doctorate in 1920 only, under the direction of Nernst on Einstein s law of photochemical equivalence. He became di-... 

The threshold for complete photochemical equivalence is calculated to be at about 3800 A. 

The photochemical equivalence law applies only to the absorption of primary photochemical process. As a result of the primary absorption only one molecule decomposes, and the products enter no further reaction, the number of molecules reacting will be equal to the number of photons absorbed. More frequently, however, a molecule-activated photochemically initiates a sequence of thermal reactions as a result of which several or many reactant molecules may undergo chemical change. Under such conditions there will be no reaction between reacting... 

In the primary process, only one molecule of HI is decomposed to its elements by absorbing one photon of light and this is in accordance to Einstein Law of Photochemical Equivalence. Subsequently, the obtained H atom reacts with another HI molecule yielding an atom of iodine. The iodine atoms obtained in steps (i) and (if) combine to give a molecule of iodine. Thus the overall effect is that 2 molecules of HI undergo chemical change by absorption of one photon of light, Hence < ) = 2... 

Homoannular dienes e.g. i) readily add excited oxygen in a reaction which is the photochemical equivalent of the Diels-Alder diene-addition reaction fyjJ. The resulting epidioxides e.g. 2) are sufficiently stable for isolation/although they are fairly reactive [72]. 

Lewis and Holman report what appears to be the first example of divergent reactions from two almost isoenergetic excited singlet states and in the photoaddition of 1-cyanonaphthalene to 1,2-dimethylcyclopentene. This is of course the photochemical equivalent of dual fluorescence. 

The Stark-Einstein law of photochemical equivalence is in a sense simply a quantum-mechanical statement of the Grotthuss-Draper law. The Stark-Einstein law (1905) is another example of the break with classical physics. It states that each molecule which takes part in the photochemical reaction absorbs one quantum of the light which induces the reaction that is, one molecule absorbs the entire quantum the energy of the light beam is not spread continuously over a number of molecules. 

Image-recording systems based on photochemical processes whether in classical silver halide or in organic nonconventlonal materials have not only to take care of the quantum yield in the sense of the Stark-Elnsteln law of the photochemical equivalent, but they must also consider the detective quantum efficiency or DQE because they can be affected by several kinds of fluctuations. 

A photochemical equivalent of this type of fragmentation may be seen in the photolysis of 2-aza-2 -deoxyadenosine 65, generating 5-amino-4-cyano-l-imidazolyl-2-deoxy-P-D-ribofuranoside 68. The authors consider two plausible intermediates, namely 66 and 67 (Equation 8). The structure of 68 was confirmed by independent synthesis < 1991 LA695>. 

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