Light quanta

Liapounov theorems for stability and instability, 346, 347, 348 Lienard, A., 334 Ufskitz, E. M., 726,759,768 Light quantum hypothesis, 485 Limit cycles, 328 coalescence of, 339 stable, 329 unstable, 329 Lindsay, R. ., 4,47 Lindstedt method of eliminating secular terms, 349... 

Absorption of a light quantum leads to an electron-hole pair Eq. (19). The electron reacts with an adsorbed oxygen molecule Eq. (20), and the hole semi-oxidizes a sulfide anion at the surface Eq. (21). Further oxidation of the sulfide anion occurs by O and O2 Eq. (22). The number of Cd ions formed equals that of the sulfate anions The oxidation of illuminated CdS powders was investigated by measuring the consumption and by detecting the superoxide radical,, by an ESR spin trapping method... 

The dismutation (disproportioning) of two free radicals is accompanied by release of a portion of reaction energy as a light quantum. As the quantum yield of such a process is extremely low, the detection of this type of chemiluminescence is technically complicated. Several compounds like lucigenin and luminol have a high quantum yield after reaction with peroxide radicals. Therefore, they are widely used for the detection of these radicals, particularly in the examination of phagocyting cells. 

The result of this collaboration was the repetition by Mark and his associate, Hartmut Kallmann, of A. H. Compton s famous confirmation of Einstein s light-quantum hypothesis (15,16). 

THE IMPULSE MOMENT OF THE LIGHT QUANTUM By Arthur Edward Ruark and Harold C. Urey Yai e University and Johns Hopkins University Commimicated September 19, 1927... 

For some years it has been customary to treat the light quantum as a particle and to emphasize those characteristics of the electron which point to its wave-like character. This idea, beautifully substantiated by the success of wave dynamics, has led Beck to assert that for each property of the electron we may hope to find a corresponding property of the light quantum, and vice versa. The spin of the electron, discovered independently by Goudsmit and Uhlenbeck and by Bichowsky and Urey, should then have its analogue in the structure of the quantum. We feel that this idea presents difiiculties, but we shall develop some of its consequences and shall discuss available data which have a bearing on it. 

These kinetic data suggest a pathway in which the nitrophenyl ester or ether, brought into an excited state by absorption of a light quantum, reacts in a bimolecular process with the nucleophile or returns to the ground state of the original molecule. At high nucleophile concentrations every excited molecule has one or more encounters with the nucleophilic reaction partner and the... 

Fig. 33. Three-halves power law dependence of the photoemission current to the power 2/3 on the energy of light quantum for a given potential (a) and on the potential for a given energy of light quantum (b). p- type gallium arsenide (/VA = 6 x 1018 cm-3) in aqueous solution. [From Boikova el al. (1976).]...

The occurrence and deactivation of excited states of the first type are schematically shown in Fig. 35. Let the minority carriers (holes) be injected into the semiconductor in the course of an electrode reaction (reduction of substance A). The holes recombine with the majority carriers (electrons). The energy, which is released in the direct band-to-band recombination, is equal to the energy gap, so that we have the relation ha> = Eg for the emitted light quantum (case I). More probable, however, is recombination through surface or bulk levels, lying in the forbidden band, which successively trap the electrons and holes. In this case the excess energy of recombined carriers is released in smaller amounts, so that hco < Eg (case II in Fig. 35). Both these types of recombination are revealed in luminescence spectra recorded with n-type semiconductor electrodes under electrochemical generation of holes (Fig. 

R with the emission of a light quantum. If the electron could pass directly into the ground state (as is usually the case with metal electrodes), no luminescence would occur. However, in the case of a semiconductor electrode the ground state is often located in the band gap, so that the corresponding electron transition is forbidden. 

Fig. 18. Influence of molecular (B) and atomic (C) oxygen on a Pt surface covered with H atoms, investigated by the change of photoelectric emission. Ordinate photoelectric yield I in electrons per light quantum. X = 302.2 rn/i T = 293°K. (a) O2 admitted, po2 = 0.34 mm. Hg (6) O2 pumped off (c) O2 admitted, poz = 10 mm. Hg, Pt spiral kept at yellow heat for 2 min. (d) O2 pumped off [according to (58)).

Reabsorption of a light quantum emitted from one activator center by another emission center... 

An electron torn away from a molecule, M, of a matrix (or an additive) by a y-quantum, by a secondary electron, or by a light quantum [reaction (1)1 is thermalized, i.e. slowed down to the rate of thermal motion [process (2)] and is then captured by a trap T [reaction (3)]. For the electron to be stabilized in the trap the energy level in this trap should be lower than the botton of the matrix conduction band (Fig. 1). The experimental investigations carried... 

It has been noted by Potasek [105] that electron tunneling in the donor-acceptor pair D-A may lead to the appearance of a charge transfer band in the absorption spectrum of this pair. The author obtained the following formula describing the dependence of the extinction coefficient, , of this band on the energy, E, of the absorbed light quantum... 

The eye is an extraordinarily sensitive instrument. To be sure, its wavelength response is restricted to 400-800 nm, but its degree of sensitivity is such that a fully dark-adapted eye can clearly detect objects in light so dim as to correspond to a light input over the retina of only about 10,000 quanta per second —one light quantum per three minutes per receptor cell in the retina ... 

Fluorescence is a three-stage process excitation via the absorption of a radiation, excited state for a very short time -(10 9 sec), and emission. The excitation of a molecule is achieved by the absorption of a light quantum of an appropriate wavelength, promoting, in a simplified view, a ir or nonbonding (n) electron to a 7r antibonding orbital. The quantum yield is a fundamental molecular property that describes the ratio of the number of emitted photons to the number of photons absorbed. 

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