Photothermal reactions

Polypropylene. In the thermal and photothermal degradations of PP as prepared as well as In the thermal degradation of the pre-irradiated polymer, the general patterns of volatile products are similar and are qualitatively accounted for In terms of the mechanism of Tsuchlya and Suml ( ), which proposed that the complex mixture of saturated and unsaturated hydrocarbons Is formed in a free radical process in which Inter and Intramolecular transfer play a predominating role. In the photothermal reaction, however, in addition to those products, a very much higher relative concentration of methane and an appreciable amount of... 

Of course an intense laser pulse will be absorbed by the surface and may cause a significant temperature rise, leading to photothermal reactions. Since the main interest is in the photochemistry it is necessary to separate the two contributions. Fortunately expressions for the temperature rise at the surface for a given laser pulse shape have been derived.18,19 Thus the temperature rise can be calculated and, if the thermal activation energy of the process under study is known or measured, the expected thermally equilibrated values of flight time and internal... 

Photoisomerization Photopolymerization Photofragmentation Photo-oxidation Photoinduced hydrolysis Photothermal reactions... 

Industrial flares and plumes represent a potentially significant source of air pollution that are poorly characterized and controlled. Conditions are ideal for post-flame thermal reactions and photolytic reactions at elevated temperatures (photothermal reactions). The elevated temperatures in flares and plumes (—50 to 600 °C outside of the visible flame) can result in accelerated rates of formation of oxy-PAH and nitro-PAH. At higher temperatures within the flame zone of combustors, oxy-PAH and nitro-PAH are likely to be destroyed, but under the relatively mild conditions of flares and plumes, the rates of formation can be accelerated without their subsequent destruction. The elevated temperatures and exposure to solar radiation can result in fast photothermal reactions that lead to the formation of both combustion-type pollutants and photochemical pollutants. Research has shown that at elevated temperatures, the rate of absorption of solar radiation and photochemical quantum yield can increase up to tenfold. 

Deterioration of paint films is promoted by photolysis, photo-oxidation, or photothermal reaction as a result of exposure to natural light. Specifically, UV light (wavelength 40-400 nm) decomposes some polymer structures. Polymer films, such as vinyl chloride resins, are gradually decomposed by absorbing the energy of UV light. 

Photothermal Reactions. The argument in the previous paragraph indicates another possible way in which light energy could be used, i.e., as a source of thermal energy to overcome activation barriers. A good example of this is provided by the reactions of carbene, formed by photolysis of diazomethane or ketene in the gas phase. 

The thermal degradation of polypropylene is accelerated by 253.7 nm radiation (photothermal degradation), and methane and ethane appear as additional products [822] these are associated with photoinitiation of the radical process involved, as the mechanisms of the thermal and photothermal reactions are almost identical. The different overall activation energies of the two reactions, 50.1 and 11.7kcal/mol, respectively, may be associated with the different initiation steps. 

Coatings for photothermal solar converters with high infrared reflectance, which use Reaction (3) and a 1000°C anneal,... 

Photothermal decomposition of palladium acetate by scanned cw Ar+ laser irradiation produces metal features that exhibit pronounced periodic structure as a function of laser power, scan speed, substrate and beam diameter, as shown in Figures 3 and 4. The periodic structure is a function of the rate at which the film is heated by absorption of the incident laser radiation coupled with the rate at which the heat of the decomposition reaction is liberated. This coupling generates a reaction front that outruns the scanning laser until quenched by thermal losses, the process to be repeated when the laser catches up and reaches unreacted material. Clearly, such a thermal process is also affected by the thermal conductivity of the substrate, the optical absorption of the substrate in those cases where the overlying film is not fully absorbing,... 

Photons, therefore, provide the activation energy needed for the initiation of a reaction. Whether or not the drug itself will absorb energy can be deduced from an examination of its UV-VIS spectrum. Since the incident photonic energy may be converted to heat, a photolytic reaction may be accompanied by a thermal reaction, resulting in a photothermal effect. Once a thermal reaction is initiated, it may proceed even in the dark (25). 

Reactions of Halo Compounds. - Calculations have been carried out to investigate the decomposition paths for methyl fluoride and methyl chloride. Methyl chloride undergoes photodissociation on irradiation at 157.6 nm. Photodissociation of methyl iodide at 266 nm has been studied. The methyl radical recombination has been followed by time-resolved photothermal spectroscopy. Methyl iodide also undergoes photochemical decomposition on a GaAs(llO) surface. " Photolysis of methyl iodide at 236 nm in the gas phase brings about liberation of iodine atoms with a quantum yield of 0.69. ... 

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