Stark-Einstein law of photochemical

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

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. 

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. 

Until now we had been talking of gas reactions. Many substances undergo photochemical reactions in liquid state. Again the reaction in initiated by Stark-Einstein law by direct light absorption on the part of reactants. However, it maybe anticipated that quantum efficiency of these reactions will be less than for the same reaction in the gas phase. The reason for this is that in the liquid state an active molecule may readily be deactivated by frequent collisions with other molecules. Furthermore, because of the very short mean free path in the liquid phase free radicals or atoms when formed photochemically will tend to recombine before they have a chance to get very far from each other. The net effect of these processes will be to keep the quantum yield relatively low. In fact, only those reactions may be expected to proceed to any extent for which the primary products of the photochemical act are relatively stable particles. Otherwise the active intermediates will tend to recombine with the solvent and thereby keep the yield low. 

If we define the primary act of the photochemical reaction as the absorption of the quantum, then the quantum efficiency for the primary act is, by the Stark-Einstein law, equal to unity. For each quantum absorbed, one primary act occurs. For any substance X taking part in a photochemical reaction, the quantum efficiency or quantum yield for the formation (or decomposition) of X is 0x and is defined by... 

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. 

When a molecule or ion absorbs a photon, that photon s energy can be dissipated in several different ways, but one way is for that energy to cause a chemical reaction to occur. The first law of photochemistry is that a compound must absorb light for a photochemical reaction to occur (Grotthuss-Draper law). The second law of photochemistry is that each photon that is absorbed activates only one molecule for a subsequent reaction (Stark-Einstein law). The quantum yield ( ) is defined as the number of molecules that react divided by the number of photons absorbed. It can also be defined in terms of moles. 

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. 

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. 

The generalizations about photochemical reactions emerged in the 1950s are perhaps too narrow. The key photochemical steps, as shown in Sect. 7.3, involve absorption of one photon (according to the Stark-Einstein law, 10 -10 s ), internal conversion, and/or intersystem crossing down to the lowest excited singlet or the lowest triplet (Kasha-Vavilov rule, or ki c up to 10 s ) and emission or... 

The photochemical Stark-Einstein law implies that the number of primary steps of a photochemical reaction must be equal to that of light quanta absorbed. Denoting the number of primary steps by ANq, the total amount of absorbed radiant energy by AI, and the light quantum by hv, this law can be written in the form... 

According to the Stark-Einstein law, (p should be equal to 1. In practice, in most cases, it is less than 1. For instance, if the quantum yield is 0.01, then only one hundredth of the molecules that are excited undergo photochemical reaction. In chain reactions, secondary processes occur and hence their cp is greater than 1. For example, in the photo dissociation of acetone, quantum yield of the reaction may be 1 or 2 depending on the number of bonds broken. 

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. ... 

Einstein photochemical equivalence law PhiYS 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-Einstein law Tn.stTn fOd O kem-a kal i kwiv a lans, 16)... 

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. 

The first law of photochemistry, named the Grotthus-Drapper Principle, states that for a photochemical reaction to occur, the first event must be the absorption of light by some component of the system. The second law of photochemistry, named the Stark-Einstein Principle, states that a molecule can only absorb one quantum of radiation. The absorbed energy in the resultant excited molecule may be dissipated by either photophysical or photochemical processes. It is the latter of these that eventually changes the chemical and mechanical properties of the substance (26,27). Thus, the reactions based on the absorption of radiation by the chemical components of modern papers are of prime importance in discoloration. 

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... 

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