Einstein law of photochemical

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. 

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... 

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. 

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. 

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. 

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 fundamental law of modern photochemistry is the law of photochemical equivalence proposed by Einstein, according to which each molecule taking part in a photochemical reaction first absorbs one quantum of energy (hv) corresponding with the frequency (v) of the radiation absorbed. (Some approach to this conception had been made by Stark in 1908.) This is the primary process in every photochemical reaction. 

In order for a photochemical reaction to occur the radiation must be absorbed, and with the advent of the quantum theory it became possible to understand the relationship between the amount of radiation absorbed and the extent of the chemical change that occurs. It was first realized by A. Einstein (1879-1955) that electromagnetic radiation can be regarded as a beam of particles, which G. N. Lewis (1875-1940) later called photons each of these particles has an energy equal to /iv, where v is the frequency of the radiation and h is the Planck constant. In 1911 J. Stark (1874-1957) and independently in 1912 Einstein proposed that one photon of radiation is absorbed by one molecule. This relationship, usually referred to as Einstein s Law of Photochemical Equivalence, applies satisfactorily to electromagnetic radiation of ordinary intensities but fails for lasers of very high intensity. The lifetime of a moleeule that has absorbed a photon is usually less than about 10 sec, and with ordinary radiation it is unlikely for a molecule that has absorbed one photon to absorb another before it has become deactivated. In these circumstances there is therefore a one-to-one relationship between the number of photons absorbed and the number of excited molecules produced. Because of the high intensity of lasers, however, a molecule sometimes absorbs two or more photons, and one then speaks of multiphoton excitation. 

Allmand AJ (1926) Einstein s law of photochemical equivalence. Introductory address to part... 

Weigert F, Brodmann L (1926) Confirmation of the Einstein law of the photochemical equivalent in a very simple photochemical reaction. Trans Faraday Soc 21 453-458... 

II) Law of Photochemical Equivalence (Ilnd law of photochemistry). This law was given by Stark in 1909 and in 1913 by Einstein. This states that. 

High quantum yield. According to Einstein s law of photochemical equivalence, one photon is absorbed by one molecule to induce a chemical transformation of only this molecule. If the quantum yield is d> > 1, this indicates the photoinitiated chain process. For the first time O 1 (up to 10 ) was observed by M. Bodenstein for the reaction of CI2 with H2 (1913). 

The second law is the Stark—Einstein law. whose re-statement in current terminology is that the primary photochemical act involves absorption of just one photon by a molecule. This holds true for the vast majority of processes exceptions to it arise largely when very intense light sources, such as lasers, are employed, and the probability of concurrent or subsequent absorption of two or more photons is no longer negligible. 

The rate of production of bromine atoms by light is estimated on the basis of Einstein s law, which requires one molecule of bromine to be dissociated for each quantum of light absorbed. In the stationary state the number of bromine atoms recombining thermally in unit time is equal to this rate of photochemical formation. Thus the number of bromine atoms which recombine per second at a known atomic concentration is found. In this way Bodenstein and Liitkemeyer find that about one collision in a thousand between bromine atoms results in combination. This number is of the right order of magnitude only, since the estimation of the number of light quanta absorbed was not very certain, and a value based only on analogy had to be assumed for the diameter of the bromine atom. 

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