Gaseous product, quantum yield

CO and sulphur are the end products of photolysis. The quantum yield of CO is 1.8, independent of COS pressure, for the photolysis at low light intensities of pure, gaseous COS at 2527 A and at 2288 A94. This value is exactly halved by the addition of a large excess of olefin, a scavenger for both ground-state and excited sulphur atoms93, suggesting that the primary quantum yield of S and CO is 0.9... 

In static systems hydrogen and mercuric oxide were found as decomposition products (59,73). Hydrogen peroxide formation was not investigated. The quantum yield for a water vapor pressure of 8.5 mm. varied from 0.02 at 45°C. to 0.04 at 580°C. (59). In flow systems about 27% oxygen was found in the gaseous products, but no hydrogen peroxide was found (9). The flow system results have been substantiated in a more recent study (7) in which the quantum yields were found to be comparable to those obtained in static systems (59). 

With the aid of a dynamic apparatus, Bowen and Tietz studied the photochemical oxidation of gaseous ethanal at around 25°C. and for near-unity values of the ratio of concentration [OjJ/IRCHO]. The primary molecular product of the reaction is, as in the liquid phase, peracetic acid. The rate of this photooxidation is independent of the oxygen concentration and is proportional to the concentration of aldehyde and to the square root of the absorbed light intensity la- The total quantum yield of the reaction is of the order of magnitude of 100. ... 

Zelikoff and Aschenbrand found that the gaseous products included benzene, diacetylene, hydrogen, ethylene and vinylacetylene. These authors photolysed C2H2 at pressures from 2 to 75 torr at 1849 A. The quantum yield of benzene formation increases markedly with pressure. Although the quantum yield for the disappearance of acetylene also increases with pressure it remains very much larger than the quantum yield of the sum of the products. The quantum yields of di-... 

At the time, the real existence of monovalent nitrogen may have been in doubt, but support for the nitrene hypothesis was soon to be forthcoming. Beckman and Dickenson identified H2, Nj and NH3 as the main products of the photodecomposition of hydrazoic acid and noted the absence of any signs of a chain reaction in the process. Irradiating gaseous HN3 with a mercury line at 199 nm they found that independently of pressure the quantum yield of nitrogen evolution was 3 0 + 0-5. This led Beckman to propose the following reaction... 

When polystyrene films are exposed to ultraviolet radiation of 235.7 nm in vacuo, crosslinking occurs and the sample becomes partly insoluble [41]. Grassie and Weir [42] have performed a detailed investigation of the photolysis of polystyrene in vacuo as a preliminary step to the study of the photo-oxidation of that polymer. They found that the only gaseous product formed is hydrogen with a quantum yield of 4.3 x 10 2. The rate of the reaction increases linearly with the intensity of the incident light... 

Polyvinylacetate undergoes crosslinking and chain scission simultaneously when irradiated in vacuo with 253.7 nm radiation. The quantum yields of both processes have been determined to be, respectively, 4.7 x 10 2 and 6.6 x 10-2 at 33°Cby measuring the soluble fraction as a function of radiation dose [8]. Gaseous products related to the acetate side group are evolved during the photolysis. Their quantum yields of formation have been estimated by vapour phase chromatography and collected in Table 2 [13]. The main volatile product, acetic acid, is... 

All data obtained for TP strongly suggest that photochemical reactions play an important role during UV laser ablation, but also that photothermal processes are important. This is confirmed by the presence of the thermal N2 products in the TOF curves. Photothermal processes will also always be present if the polymer decomposes exothermically during a photochemical decomposition and if the quantum yields of the photochemical reaction is not equal to one (which is most of time the case). The ablation of polymers will therefore always be a photophysical process (a mixture of photochemical and photothermal processes), where the ratio between the two mechanisms is a function of the irradiation wavelength and the polymer. In addition, photomechanical processes, such as pressure produced by trapped gaseous ablation products or shock and acoustic waves in the polymer, take place and can lead to a damage of the polymer and are most important for picosecond pulses. 

Sometimes the quantum yields are determined for gases or liquids of low molecular weights which are formed during the photodegradation of polymers. For example the quantum yield of gaseous product (( g) is defined... 

The main gaseous products formed from the Norrish Type I reaction are acetaldehyde, carbon monoxide and methane, which are produced with quantum yields 0.06,0.003 and 0.0006 respectively, by the following reactions [2283] ... 

The main gaseous products formed during photolysis of poly(vinyl acetate) are acetic acid, acetic aldehyde, carbon monoxide, carbon dioxide and methane [335, 336, 749]. Quantum yields of formation of some of these products are given in Table 3.8. 

UV irradiation of poly(methylphenylsiloxane) (4.114) [1986], poly(hexyl-methylsilane) (4.115) and poly(cyclohexylmethylsilane) (4.116) [579] causes chain scission and crosslinking processes. In the case of poly(methylphenyl-siloxane) (4.114) very small amounts of different gaseous products with low quantum yields ((/>h2 = 2.6 x 10 0ch4 = O-63 x 10 0c2H6 = O.12 x 10 and c2H2 = 3.06 x 10 ) are formed [1986]. 

The exact mechanism of activation of carbon dioxide is still unrevealed. Great research efforts are needed to overcome this bottleneck problem in terms of both experimental and computational research. Quantum mechanical modelling gives a better insight to the activation mechanism. Once one identifies the appropriate material which will adsorb even gaseous carbon dioxide and transfer it into the corresponding radical anion, yield products with greater than 10% silver bottom efficiency, in a similar way of ammonia synthesis, then the rest becomes history. 

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