Decomposition, dependence light intensity

Torimoto, T. Aburakawa, Y. Kawahara, Y. Ikeda, S. Ohtani, B. Chem. Phys. Lett. 2004, 392, 220 This paper showed that the rate of photocatalytic reaction in the presence of molecular oxygen, i.e., photocatalytic oxidative decomposition, may strongly depend on the intensity of light irradiation, while the rate of reaction in the absence of oxygen seems to be almost independent of light intensity. 

We prefer to explain the results of Herr and Noyes as well as those of Anderson and Rollefson on the pressure effect by the pressure dependence of the decomposition of the CH3CO radical. According to the Benson equation , based on the detailed theory of unimolecular reactions, the acetyl radical is in the fall-off region below a few atmospheres. O Neal and Benson demonstrated that the results, obtained in the absence of added foreign gases at low light intensities - , are in accordance with the assumption of a pressure-dependent decomposition and a homogeneous recombination of the acetyl radicals. 

Decomposition step (9) is strongly supported by the above results. The high triplet yields found at 3650 A led us to the conclusion that step (10) also plays a significant role at this wavelength. The considerable temperature dependence of the phosphorescence shows that the reaction has a high activation energy 13, u 9,242 Since, at 3650 A, the primary quantum yields are independent of the light intensity... 

Thus, it has been studied azoethane decomposition [21] stimulated by ruby laser (/.=694.3nm) with power density equal to 70+175 MW/cm2. It has been established that the N2 yield is proportional to the order 2.2 0.1 with respect to the light intensity. The authors of [22] suppose that after photon absorption the excited molecules from the first excited singlet state A are transfered to an other excited singlet state B from which fluorescence is forbidden. The both states are at close range. The excited molecules which are on B level may be decomposed with a rate which depends on the nature of radicals linked with azo group. The authors of [23] assume that both mechanisms (thermo-and photo) are identic, but only in the case of photolysis transfer to triplet (T) state is possible. 

The rates of initiation depend on the type of activation ch.osen. In photochemical initiation, Vs = 2 0/, where I is the absorbed light intensity and 0 = the efficiency coefficient. With an average intensity, rates of initiation of approximately 10"7 mole l 1 s 1 are attained. In thermal activation, autoinitiation by interaction between oxygen and aldehyde gives low values of approximately 10 9 mole l-1 s"1 under standard laboratory conditions. When azonitrile is used, since the thermal decomposition rate of this product is approximately first order [62], Vi is given by... 

The influence on the rates due to light intensity, wavelength, and sample purity requires that well-characterized samples be photolyzed at discrete wavelengths over a broad spectral region. The dependence of rate on intensity must also be measured at each wavelength. With this information, a spectral decomposition curve can be obtained and more meaningful comparisons made between compounds. 

Polypropylene.—The photo-induced decomposition and stabilization of polypropylene has been the subject of numerous papers. The rate of photo-oxidation has been reported to be proportional to fractional powers of the light intensity, depending on the range of wavelengths used and the presence of stabilizers. Vink, in the report of a comprehensive kinetic study, proposed that the mechanism... 

DEPENDENCE OF THE EQUILIBRIUM DECOMPOSITION RATE ON LIGHT INTENSITY... 

Photoinitiators are seldomly used in commercial applications, because large reaction volumes cannot be irradiated easily in a uniform way. Still, there are some applications in the area of UV hardening lacquers and printing inks. In research, on the other hand, photoinitiators are used frequently, for they allow to precisely define the beginning and end of the initiation process by flash photolysis of the initiator. Also, most photoinitiators show almost no temperature dependence of the decomposition rate, but a strong dependence on the (UV) light intensity. 

Mislow and co-workers (258) and Hammond (259) have shown that optically active diaryl sulfoxides, which are configurationally stable in the dark at 200°C, lose their optical activity after 1 hr at room temperature on irradiation with ultraviolet light. Similarly, an easy conversion of the trans isomer of thianthrene-5,10-oxide 206a into the thermodynamically more stable cis isomer takes place upon irradiation in dioxane for 2 hr. However, the behavior of a-naphthylethyl p-tolyl sulfoxide under comparable irradiation conditions is different, namely, it is completely decomposed after 4 min. These differences are not surprising because the photochemical racemization of diaryl sulfoxides occurs by way of the pyramidal inversion mechanism whereas decomposition of the latter sulfoxide occurs via a radical mechanism with the cleavage of the sulfur-carbon bond. It is interesting to note that photoracemization may be a zero-order process in which the rate depends only on the intensity of the radiation and on the quantum yield. 

The use of silver bromide (or, less commonly, silver chloride or iodide) depends on the fact that these salts, when exposed to light, undergo decomposition to an extent that is dependent on the intensity of the light. The nature of the chemical changes involved in the simplest type of photographic process may best be indicated in terms of the following steps ... 

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