Photoproduct accumulation

In the simplest case, with successive laser pulses, photoproduct accumulation in the substrate may be described by an exponential of the form [1-ex-p(—crFlaserNpulse) ] > where a is the absorption cross-section and Npuise is the number of pulses, until finally a plateau is reached. Indeed, this dependence is largely borne out in the experiments on the model, doped systems. Furthermore, examination of the probe spectra from samples that have been irradiated at various fluences but at approximately the same total photon dose (Fig. 16) indicates an interesting result. At all fluences, the photoproduct... 

A rather important aspeet that should be eonsidered is that interfaeial quenching of dyes does not neeessarily imply an eleetron-transfer step. Indeed, many photoehemieal reactions involving anthracene oeeur via energy transfer rather than ET [128]. A way to discern between both kinds of meehanisms is via monitoring the accumulation of photoproducts at the interfaee. Eor instance, heterogeneous quenehing of water-soluble porphyrins by TCNQ at the water-toluene interfaee showed a elear accumulation of the radical TCNQ under illumination [129]. This system was also analyzed within the framework of the exeited-state diffusion model where time-resolved absorption of the porphyrin triplet state provided a quenehing rate eonstant of the order of 92M ems. ... 

No mirex degradation products were detected in whole fathead minnow or in hydrosoils under aerobic or anaerobic conditions (Huckins et al. 1982). In contrast, three metabolites were detected in coastal marshes after mirex bait application, one of which, photomirex, was accumulated by fish and oysters (Cripe and Livingston 1977). The fate and effects of mirex photoproducts in the environment are unclear and merit additional research. 

E. Hustede, A. Steinbuchel, H. G Schlegel (1993) Relationship between the photoproduction of hydrogen and the accumulation of PHB in non-sulphur purple bacteria, Appl. microbiol. Biotechnol., 39 87-93... 

Photo-oxidation leads to the formation of carbonylic products and this is classically monitored by vibrational spectroscopy. To investigate the relation between the accumulation of the oxygenated photoproducts and the change in the crystallinity of polycyclooctene, the decrease of the heat of crystallization was compared with the rise of the concentration of carbonyl function (1721 cm 1 band) as displayed in Figure 10.9. The enthalpy of crystallization falls at early stages of irradiation before significant accumulation of the carbonyl. Assuming that the decrease of the... 

Photoproducts resulting from the oxidation of the styrenic units (such as benzoic acid) accumulate in ABS at long irradiation times. 

The photoproducts resulting from the oxidation of the styrene and acrylonitrile units accumulated at longer irradiation times (aliphatic and aromatic ketones and acids, amides, imides, etc.). 

In the AES and PS-PVME blends, no interaction in terms of nature or reactivity of the photoproducts formed was detected. All the blends contained an elastomeric phase which has been shown always to be the most oxidizable component. EPDM and PVME degrade in the first few hours of irradiation and the photoproducts resulting from the oxidation of SAN or PS accumulate at longer irradiation times. However, the styrene units are oxidized faster in the blends than in the homopolymer PS or in SAN. In addition, in the PVME-PS blends rich in PS polymer, the PS retarded the photodegradation of PVME. 

Grellmann and co-workers, who have contributed considerably to this area with their work on the photocyclization of diarylamines to carbazoles, now report that the only unknown isomer (157) of the five possible indolocar-bazoles can be conveniently synthesized from (158) in methylcyclohexane solution by using 300 nm radiation.Flash experiments and steady-state studies show that the formation of (157) occurs adiabatically in the triplet manifold by a two-photon process and a triplet energy transfer reaction prevents accumulation of large concentrations of the intermediate photoproduct (159). Photocyclization of the aryl-ethenyl compounds (160) has been... 

High-energy input into the solution (more than 10 mj) can lead to rapid accumulation of undesired photoproducts. Thus, it is better to use a short (1-2 nsec), less intensive pulse (0.5—2 mj) than massive (5—50 mj) longer ones (5—20 nsec). 

Fig. 15 Pulse evolution of the intensities of C6H5C1 and of the various photoproducts formed in the 248 nm irradiation of condensed films of C6H5C1 (Flaser=150 mj/cm2, whereas the ablation threshold is 100 mj/cm2). It is clear that HC1 is nearly constant with successive laser pulses, whereas for the other indicated species, accumulation with successive laser pulses is significant, until finally an equilibrium between amount of photoproduct formed and photoproduct ejected is attained. A more detailed description of the observed effects is given in [35]...

Ablation with femtosecond pulses is comprehensively reviewed by Kruger and Kautek in this issue. The present discussion focuses exclusively on photoproduct formation in the irradiation of Arl-doped systems with fs pulses. This examination indicates several subtle mechanistic possibilities. Upon irradiation with 500-fs pulses at 248 nm, only ArH-like product formation is observed as the fluence is raised above the ablation threshold (Fig. 17). Recombination (e.g., Nap2-type) or other by-products are not observed even for dopant concentrations as high as 4 wt% [83-84]. In contrast, at low flu-ences, a number of by-products are observed after irradiation with few laser pulses. In this case, the accumulating radicals evidently react with each other to produce ill-defined products. Most interestingly, ill-defined species are not observed above the threshold even after extensive irradiation (Fig. 17). It is clear that photochemical processes in the ablation with fs pulses must differ distinctly from those in ns ablation. 

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