Photodecomposition, energy

Another method, called photobleaching, works on robust soHds but may cause photodecomposition in many materials. The simplest solution to the fluorescence problem is excitation in the near infrared (750 nm—1.06 pm), where the energy of the incident photons is lower than the electronic transitions of most organic materials, so fluorescence caimot occur. The Raman signal can then be observed more easily. The elimination of fluorescence background more than compensates for the reduction in scattering efficiency in the near infrared. Only in the case of transition-metal compounds, which can fluoresce in the near infrared, is excitation in the midvisible likely to produce superior results in practical samples (17). 

Anotlrer consideration in the production of thin fllms by photochemical processes is that the fraction of the beam which is not used in photodecomposition will heat any substrate on which it is desired to form the fllm. The power of tire light source which can be used for photodecomposition in the gaseous phase only is therefore limited by the transmission of energy. Clearly this transmitted beam represents a constant source of energy which... 

Dacarbazine is activated by photodecomposition (chemical breakdown caused by radiant energy) and by enzymatic N-demethylation. Formation of a methyl carbonium ion results in methylation of DNA and RNA and inhibition of nucleic acid and protein synthesis. Cells in all phases of the cell cycle are susceptible to dacarbazine. The drug is not appreciably protein bound, and it does not enter the central nervous system. 

Trigonal, metallic selenium has been investigated as photoelectrode for solar energy conversion, due to its semiconducting properties. The photoelectrochemistry of the element has been studied in some detail by Gissler [35], A photodecomposition reaction of Se into hydrogen selenide was observed in acidic solutions. Only redox couples with a relatively anodic standard potential could prevent dissolution of Se crystal. 

Fig. 7.1 Position of band edges and photodecomposition Fermi energies levels of various non-oxide semiconductors. E(e,d) represents decomposition energy level by electrons, while E(h,d) represents the decomposition energy level for holes vs normal hydrogen electrode (NHE). E(VB) denotes the valence band edge, E(CB) denotes the conduction band edge. E(H2/H20) denotes the reduction potential of water, and (H2O/O2) the oxidation potential of water, both with reference to NHE.

The photodecomposition of -alkanes at excitation energies slightly above the absorption onset involves both C-H and C-C bond decompositions [18]. The dominant process is the C-H scission, (H2) 0.8-0.9, and the contribution of C-C decomposition is small. In the photolysis of cyclohexane, cycloheptane, cyclooctane, and cyclodecane, however, only hydrogen evolution was observed [[Pg.375]

It would be elegant to finish the part on photophysics and photochemistry of liquid alkanes by giving a picture that unifies the temperature- and energy-dependence results obtained in fluorescence and photodecomposition studies. However, the spectroscopic information available for alkane molecules is not sufficient to identify the exact excited states involved in the radiative and nonradiative processes [55]. Because of the lack of information, there are different views on the positions and identities of excited states involved [52,55,83,121,122]. 

The proposed mechanism may explain such excited-state characteristics as the temperature dependence of lifetime or Aex dependence of the fluorescence intensity at low excitation energies. However, in order to explain the energy dependence of the photodecomposition at high energies at least one more dissociative state should be included in the mechanism, which decompose to radicals. 

G(X) and quantum yield of the product used for the calculation, respectively. Two possible sources of errors should be mentioned. One is the energy dependence of the photodecomposition. In order to obtain intrinsic yield should be measured at photon energies as close as possible to the absorption onset, or appropriate corrections should be used [154,155]. Both photon emission and photodecomposition studies show that the energy dependence is more severe for the smaller molecules than for the larger ones. Therefore, G(Si) for the larger molecules can be estimated with higher accuracy. The second source of error is that X may not solely form in Si molecule transformation. 

The (CH3CO-CH3) bond dissociation energy has been measured by the kinetic, and more recently by the electron impact, methods, and concordant values40 41 of 71 kcal. mol.-1 were obtained so that when acetone suffers a photodecomposition of type A, the fragments still carry excess energy. 

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,... 

While there are no reliable data from which the (CH3COCH2-H) bond dissociation energy could be estimated, it is safe to assume that it will exceed the energy available at a wavelength of 3130 A. so that a photodecomposition of type C would not be expected to occur. 

In the case of hexafluoroacetone, the C-F bond dissociation energy must be so high that any photodecomposition of type C would not be anticipated. There are no thermochemical or electron impact data available from which it would be possible to assess the relative importance of the processes of types A and B and this is unfortunate for the chemical evidence is conflicting. 

The presence of chlorine atoms in the ketone molecule leads to a further complication, since the C-Cl bond dissociation energy may be of the order of 80 kcal. mol.-1, so that a photodecomposition of type C becomes energetically feasible. 

The photodecomposition of perfluoro diethyl-62 and perfluoro-di-w-propyl63 ketones has been shown to follow a course similar to that of hexa-fluoroacetone. The fluorescence is weaker in the case of the perfluoro-diethyl ketone and almost absent in the case of the perfluorodi-n-propyl ketone, and the pressure dependence of the quantum yields of carbon monoxide is consistent with this behavior. It is proposed that in the more complex structures, the energy may more easily be accommodated and lost by internal conversion, thereby reducing the contribution made by fluorescence and by Collisional quenching of the excited molecule. 

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