Photodecomposition, energy requirements

The energy required to dissociate the acetone molecule into two methyl radicals and a molecule of carbon monoxide may be calculated from thermocheinical data and amounts to 89 kcal. mol.-1. It would be energetically possible, therefore, for a photodecomposition of type B to occur,... 

The very small photodecomposition yields at low temperatures are to be expected in this region, as the photon energy available at 365.0 nm [78.5 kcal/mol is less than that required for dissociation by process 93 and is marginal for process 92 (AH92 = 69.5 kcal/mol, AHg = 89.3 kcal/mol)]. The temperature dependence of these yields has been best explained as a unimolec-ular decomposition of T- with an activation energy of 15 kcal/ mol (176), a value comparable to Ea values found for acetone and hexafluoroace tone. 

A theoretical calculation shows that 80 kcal/mol is required to form oxirene from ketene (73). The fact that hot ground-state ketene has no low-energy decomposition pathway available may make the explanation of the pressure dependence of photodecomposition yields plausible. The thresholds for 3g and A production have been measured as 75.7 1.0 kcal/mol and 84.0 0.6 kcal/ mol, respectively (225). These high barriers should therefore make the decomposition relatively slow for a molecule this size at low excitation energies, and thus subject to pressure quenching at moderately high pressures (10-100 torr). 

The photodissociation of the parent 1,2,4,5-tetrazine was found to be exothermic by more than 100 kcal mol- 1.263 The activation energy for the photodecomposition of 1,2,4,5-tetrazine is 39.7 kcal mol-1 and 40.1 kcal mol-1 for 3,6-dimethyl-1,2,4,5-tetrazine.107 Using the results obtained a mechanism has been suggested wherein the tetrazines first absorb a photon, then decay to an intermediate species which under certain conditions requires absorption of a second photon to achieve decomposition to nitrogen and nitriles.267,273... 

The suggestion of Mott [190], that photodecomposition of Ba(N3)2 occurs by the same mechanism in silver halides, was disputed by Tompkins and coworkers on the basis of additional observations [80,191,206]. In particular, the photoconductivity was found to be too small to account for the electron motion necessary for the formation of barium colloids [80]. More recently, Marinkas and Bartram were unable to detect photoconductivity in anhydrous crystals [49]. In addition, measurements of the dark conductivity indicated that if it is due to Ba ", it is much too small to account for the observed rate of photodecomposition [80,206]. As a further indication that the photodecomposition of Ba(N3)2 does not take place by the silver hahde process, the energy of formation of a barium interstitial was estimated and found to be much greater than the estimated energy for vacancy formation, thus indicating the possibility of Schottky disorder rather than Frenkel disorder as intrinsic to Ba(N3)2 [206]. Interstitial metal ions are required for the Mott-Gurney mechanism discussed above [167]. 

Hall and Williams [96] doped thin films of lead azide with T1 and Bi. There was no marked effect on the photodecomposition efficiency at 330 nm as compared to undoped films. However, both the spectral dependence of the rate and the optical absorption were altered by thallium. The incorporation of T1 (10 mole fractions) removed the 375 nm peak from the optical absorption spectra while the incorporation of Bi left the peak unaltered. Partial decomposition of films (0.1%) also removed the 375 nm peak (dotted curve. Figure 32). The results are consistent with the fact that the Tl " impurities require anion vacancies for charge compensation. This is equivalent to partial decomposition. They concluded that the peaks in the optical absorption curve and spectral photodecomposition curves are probably a result of charge-transfer excitons. Furthermore, peak separations may arise because of differences in the interaction energies of inequivalent lead and azide ions in the unit cell. The selective removal with decomposition of the 375 nm peak may indicate selective decomposition of the azide site having the highest valence band energy. The selective decomposition would reduce the density of states and thus the extinction coefficient for electronic transitions from that particular azide band. 

To accomplish photodecomposition of water, so that the PEC cell supplies the entire potential, energetic of the cell should meet the requirements as shown in Fig. 5. It is assumed that Fermi energy of n-type semiconductor (or its flat band potential) is equivalent to water reduction potential. Alternatively, instead of using metal as a counter electrode, one could use a p-type semiconductor (Fig. 6). Because... 

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