Photochemistry second law of

Only one photon at a time may interact with matter. This means that the energy available to each recipient atom or molecule is the same as the energy possessed by the single photon with which it interacts. This truth was refined by Stark and Einstein, who called it the second law of photochemistry. If a species absorbs radiation, then one particle (molecule, ion, atom, etc.) is excited for each quantum of radiation (photon) that is absorbed . 

The second law of photochemistry says that if a species absorbs radiation, then only one particle is excited for each photon absorbed. 

It is easy to get burned by the sun while out sunbathing, because the second law of photochemistry shows how each UV photon from the sun releases its energy as it impinges on the skin. This energy is not readily dissipated because skin is an insulator, so the energy remains in the skin, causes photo-excitation, which is experienced as damage in the form of sunburn. 

Conversely, a quantum yield

greater than unity cannot be achieved during a straightforward photochemical reaction, since the second law of photochemistry clearly says that one photon is consumed per species excited. In fact, values of > 1 indicate that a secondary reaction(s) has occurred. A value of > 2 implies that the product of the photochemical reaction is consumed by another molecule of reactant, e.g. during a chain reaction, with one photon generating a simple molecule of, say, excited chlorine, which cleaves in the excited state to generate two radicals. Each radical then reacts in propagation reactions until the reaction mixture is exhausted of reactant. 

To help clarify the situation, we generally define two types of quantum yield primary and secondary. The magnitude of the primary quantum yield refers solely to the photochemical formation of a product so, from the second law of Photochemistry, the value of 0(primary) cannot be greater than unity. 

As a natural consequence of the second law of photochemistry, the sum of the primary quantum yields cannot be greater than unity. 

The second law of photochemistry was first enunciated by Stark (1908) and later by Einstein (1912). The Stark-Einstein law 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. 

This last is a reformulation of the Second Law of Photochemistry, also known as the Stark-Einstein Law. 

The second law of photochemistry (Einstein, 1905) states that light absorption is a quantum process. Usually, one photon is absorbed by a single molecule. 

The primary quantum yield (Primary Q.Y.) establishes the number of molecules degraded from a primary process or event that involves direct absoiption of radiation over the number of photons absorbed (Cassano et al., 1995 and Davydov et al., 1999). Cassano et al., (1995) argue that according to the second law of photochemistry, the absorption of light by a molecule is a one-quantum process. Therefore a quantum yield factor involving the sum of all primary processes must be less than or equal to unity and this as a result that the energy absorbed by the molecule is partially lost by re-emission, collision or other processes (Alfano and Cassano, 1988). 

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. 

The second law of photochemistry, formulated in 1912 by Einstein, is that one molecule of reacting substance may be activated and caused to react by the absorption of one photon. In some systems, such as material containing rather stable dyes, many photons are absorbed by the molecules for each molecule that is decomposed the fading of the dye by... 

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... 

A paper published by Einstein in 1912 [34] is usually taken as the source of the second law of photochemistry. In the opening paragraph, he states ... 

This would be followed by the second law of photochemistry which might thus be expressed as ... 

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