Photochemical equivalence, law

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 ... 

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... 

The photochemical reaction of a material starts with photon absorption. In other words, only the photons absorbed by the molecule can bring about photochemical reactions. This is the first law of photochemistry, also called the Grotthuss-Draper law. The second law of photochemistry is one molecule is activated when one photon is absorbed. This is called the Stark-Einstein photochemical equivalence law. Generally, a particular group in an irradiated molecule absorbs a photon with an appropriate wavelength. When photoabsorption occurs, the molecule in the ground state is... 

In the following the Wien irradiation law and the photochemical equivalence law will be derived through an essentially thermodynamic way. As for the latter law, I mean the statement that for the destruction of one equivalent through a photochemical process the irradiation energy Nhv is required, where iV is the number of molecules in a gram-molecule, h the known constant in the Planck irradiation energy formula and v the fiequency of the active radiation. Such law appears to be essentially a consequence of the assumption that the number of molecules destroyed per unit of time is proportional to the density of the active radiation. It should be stressed, however, that the thermodynamic dependence and the irradiation law do not allow substituting this hypothesis by a preferred one, as it will be mentioned at the end of the paper. 

The second principle of photochemistry is called the photochemical equivalence law, or the Stark-Einstein law, which states the absorption of light occurs in the quantum unit of photon or one molecule absorbs one photon, and one or less molecule can be photolyzed accordingly. ... 

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 . 

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-... 

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. 

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. 

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. 

Even with ordinary radiation it may appear that the law of photochemical equivalence is not obeyed, and this arises because of two factors. One is that a molecule that has absorbed a photon may become inactivated before it has had time to enter into reaction. When this alone is the case the ratio of the number of molecules undergoing reaction to the number of photons absorbed (a quantity known as quantum yield or photon yield) is less than unity. The second factor is that the reaction may occur by a composite mechanism. For example, in the photochemical decomposition of hydrogen iodide (HI) into hydrogen and iodine, the quantum yield is 2 that is, one photon brings about the decomposition of two molecules of hydrogen iodide. The reason is that the mechanism is... 

Plotnikov later took pain in demonstrating [19] that taking the Grotthuss-Draper law in a quantitative sense, that is, that not only light has to be adsorbed, but that the effect is proportional to the radiation absorbed, could replace the Stark-Einstein law, but he obviously missed the point. The equivalence law states much more precisely that absorption of one quantum of light causes one photochemical act. Whether this applies in every single case or, more correctly, which is the mean probability that this statement applies to one type of acts or to another one, and under which conditions it can be affirmed that this applies to a chemical reaction, is another question (see below). That the effect is proportional to the absorbed flux— provided that the terms are properly defined—is obvious. 

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