Law of 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 . 

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. 

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... 

Taylor contrasted the most uncompromising form of this law, where an exact equivalence between absorbed quanta and reacting molecules was postulated, with experimental evidence that showed that this was only exceptionally the case. After examining the available data [75, 76], he suggested that the difficulties inherent in the acceptance of the law of photochemical equivalence as originally formulated would embody the elements of this law which have found support from its study. For this purpose it seems necessary first to avoid entirely the name which has become usual in reference to this matter, since equivalence has been demonstrated only in exceptional cases, rather than as a general rule. ... 

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

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 law of photochemical equivalence is restricted to primary photochemical process, i.e., each reacting species excited by the absorption of one radiation get chemicd transformation and formed products produce no further reaction. In such cases, these will be 1 1 relationship between the number of quantas absorbed and the number of reacting molecules. But in practice, most of photochemical reactions undergoes secondary photochemical reactions, i.e., photochemically activated species or product molecule initiates a series of chemical transformations while in some cases, photochemically activated species undergoes deactivation, they lose their energy in the form of heat or radiation. Under such conditions, there will be no more 1 1 relationship between the number of quanta absorbed and the product molecules. The deviation from photochemical equivalence (1 1 relationship) is described by the idea of quantum yield or quantum efficiency (< )). It is defined as... 

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). 

---