Light, chemical effects

Lind (1961) defines radiation chemistry as the science of the chemical effects brought about by the absorption of ionizing radiation in matter. It can be said that in 1895, along with X-rays, Roentgen also discovered the chemical action of ionizing radiation. He drew attention to the similarity of the chemical effects induced by visible light and X-rays on the silver salt of the photographic plate. This was quickly followed by the discovery of radioactivity of uranium by Becquerel in 1896. In 1898, the Curies discovered two more radioactive elements—polonium and radium. 

Furthermore, the magnitude of the energy in this answer helps explains why we sometimes wish to employ light to effect a reaction, because a chemical reagent simply does not possess enough energy (see later examples). 

We present here a condensed explanation and summary of the effects. A complete discussion can be found in a paper by Hellen and Axelrod(33) which directly calculates the amount of emission light gathered by a finite-aperture objective from a surface-proximal fluorophore under steady illumination. The effects referred to here are not quantum-chemical, that is, effects upon the orbitals or states of the fluorophore in the presence of any static fields associated with the surface. Rather, the effects are "classical-optical," that is, effects upon the electromagnetic field generated by a classical oscillating dipole in the presence of an interface between any media with dissimilar refractive indices. Of course, both types of effects may be present simultaneously in a given system. However, the quantum-chemical effects vary with the detailed chemistry of each system, whereas the classical-optical effects are more universal. Occasionally, a change in the emission properties of a fluorophore at a surface may be attributed to the former when in fact the latter are responsible. 

The inversion, with or without invertase or acid, has also been useful in studying the chemical effect of the deuterium ion, of high pressure, of sonic and supersonic energy of ultraviolet light " of high-frequency electric energy and of variation in the dielectric constant of the solvent. ... 

F Meaty, nutty, onion-like F at 0.2-1 ppm Freshly chopped, meat-like with light sour effect F at 2 ppm Chemical... 

The attraction of coloring matter by charcoal is denominated mechanical, while the attraction of sulphuric acid by baryta is classed with chemical effects. But animal charcoal does not only attract coloring matter it aho decomposes metallic salts, and fixes their base by that same power of retention., Now it is known that chemical combinations and decompositions are produced by differences in intensity of one only chemical attractive force eollod affinity. It is evident that the same force is inherent in animal charcoal. This view is corroborated by the fact thot animal charcoal absorbs different compounds in different bnt determined quantities. Niepce has shown that iodine and chlorine gas ore condensed by the Inked portion of printed paper, while the white portion of the paper dees not retain any of the gases. Charcoal absorbs heat and light most readily.. It.Condenses... 

The fact that chemical change can be induced by visible light shows that the energy of electronic activation can under appropriate circumstances find its way to the critical position in the molecule, but from what has been said it should be clear that no direct connexion exists between the magnitude of light quanta and their possible chemical effects. This statement is borne out by all the facts of photochemistry. 

Recently, liquid water was decomposed to hydrogen and oxygen stoichiometrically and continuously by irradiations of ultrasound and light with particulate photocatalyst.n) This reaction system is thought to be a joint one for sonolysis and photocatalysis. Furthermore, this system also is a hybrid of mechanical effects and chemical effects. In this chapter, the effect of ultrasound on photocatalytic reaction is considered. The joint system of sonochemical and photocatalytic reactions, in particular s explained. 

Earlier we defined the terms primary photochemical process and secondary reaction. Another term of great significance when dealing with the chemical effects of light is primary photochemical yield. This is the number of absorbing molecules which either dissociate or react per photon absorbed. The primary quantum yield may not exceed unity and it may be zero, if, for example, all absorbing molecules fluoresce without reacting (14). 

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