Decomposition, excited neutral

On the other hand, the formation of ethylene was ascribed mainly to the unimolecular decomposition of a neutral excited propane molecule. These interpretations were later confirmed (4) by examining the effect of an applied electrical field on the neutral products in the radiolysis of propane. The yields of those products which were originally ascribed to ion-molecule reactions remained unchanged when the field strength was increased in the saturation current region while the yields of hydrocarbon products, which were ascribed to the decomposition of neutral excited propane molecules, increased several fold because of increased excitation by electron impact. In various recent radiolysis 14,17,18,34) and photoionization studies 26) of hydrocarbons, the origins of products from ion-molecule reactions or neutral excited molecule decompositions have been determined using the applied field technique. However, because of recent advances in vacuum ultraviolet photolysis and ion-molecule reaction kinetics, the technique used in the above studies has become somewhat superfluous. 

The triazene polymers are also well suited as probes for the ablation mechanism. Mass spectrometry was used to study the ablation products and to determine the different ablation mechanisms at the different irradiation wavelengths [67,137,138]. All decomposition products were identified with time-resolved mass spectrometry for 248 and 308 nm irradiation. The proposed decomposition pathway for 308 nm irradiation is shown in Fig. 14.14, but similar products were observed also for a thermal decomposition [126]. A clearer indication for the presence of a photochemical mechanism for 308 nm irradiation was given by TOF-MS. Three different species of nitrogen were detected in the ablation plume a very fast ground state neutral with up to 6 eV of kinetic energy, a slower ground state species with a broad energy distribution that is most probably a thermal product, and a metastable (excited) neutral N2 species that can only be created by an electronic excitation [139]. 

Between 8 and 30 e.v. there are at least three different processes responsible for producing NH2 radicals by electron impact on NH3 molecules. Two of them are ion-molecule reactions, the primary ions being NH3+ and NH2+. Another process occurring near 18 e.v. is probably the decomposition of a highly excited neutral state of NH3. 

The relative product distribution produced by photons of 10-e.v. energy in the krypton resonance radiation photolysis will also be taken as representative of excited neutral decomposition induced by electron impact at energies exceeding the ionization potential of ethyl chloride (10.9 e.v.). That is, the contribution to the radiolysis products from excited neutral molecule decomposition will be assumed to have the same relative distribution as that observed in the photolytic decomposition. While superexcited molecules will also be produced in the radiolysis, there is considerable evidence to support the view that their modes... 

On the basis of previous discussion, the only additional source of ethylene in the radiolysis is the decomposition of excited neutral ethyl chloride molecules. If the ion-molecule contribution to this product is subtracted from the total yield reported in Table III, an ethylene-acetylene yield of G = 4.24 may be attributed to excited neutral decomposition. If we now assume that the photolysis experiments provide a direct measure of the neutral excited molecule decomposition to be expected in the radiolysis, this ethylene yield may be used as a basis for normalization to estimate the contributions from this source to the other products using the relative photolysis distributions from Table IV. In this way, the contributions from excited neutral decomposition reported in Table V were derived. 

Product Excited Neutral Decompositions Ion-Molecule Reactions and Ionic Fragmentation Neutralization... 

Ionic reactions in ethyl chloride have been studied by both mass spectrometric and radiolysis techniques. The radiolysis mechanism advanced on the basis of our experimental observations indicates that the major radiolytic reaction mode in this system is excited neutral molecule decomposition. While the role of ionic reactions in the radiolysis therefore appears to be relatively minor, it was possible to establish a good correlation between the predictions of the mass spectrometric studies with respect to ionic intermediates and the participation of such ions in the radiolytic reaction scheme. These results emphasize the advantages of combining the techniques used here to obtain a complete description of the reactive system. 

Polymer anion radicals are usually less reactive than the cation radicals, and are often stable at 77K, but they are usually unstable at room temperature. Excited state species can undergo decomposition by a variety of routes including (i) homolytic cleavage to form two neutral radicals, (ii) heterolytic cleavage to form an anion and a cation, or (iii) bond rupture with the formation of two neutral molecules. 

The dependence of rates of ionic decomposition upon emitter temperature can be attributed to changes in the internal energy of the molecular ion, i.e. vibrational excitation is temperature dependent. The temperature of the sample molecules is governed, to some extent, by the emitter temperature (vide infra) and increasing the vibrational energy of the neutral molecule results in increased vibrational energy within the molecular ion [519]. 

Knotek and Feibelman [94] examined the modification to a surface when exposed to ionising radiation and assesed the damage that can be produced. They addressed the stability of ionically bonded surfaces, where the KF mechanism applies, and concluded that Auger induced decomposition only occurs when the cation in the solid is ionised to relatively deep core levels. In the case of non-maximal oxides as with NiO, Freund s group [95] showed that whilst desorption of neutral NO and CO from NiO(lOO) and (111) surfaces has thresholds at the C Is, N Is and O Is core levels, it proceeds mainly on the basis of the MGR model, involving an excited state of the adsorbate. An overview of electronic desorption presented by Feibelman in 1983 [96] examined particularly the stability of the multiple-hole final state configuration leading to desorption. The presence of multiple holes, and associated hole-hole correlation... 

The decomposition of the neutral excited species formed by 1048-1067 A radiation was found to be the same as for the longer wavelengths of 1236 and 1470 A. The only trend which was apparent was a gradual decrease in the ratio of molecular to atomic hydrogen elimination (reaction (2)/reaction (3)) as the wavelength decreases. 

The decomposition of the neutral excited cyclopentane molecules is very similar to that at 1236 A. 

Product formation in a radiolysis system is often complex as a result of the many different species present. There are, however, three main types of reactive species excited molecules, ions and free radicals. The excited molecules and ions are generated directly, while the free radicals are formed by dissociation of the excited molecules or ions. The reactions of these three species account for the products. Decomposition or combination and disproportionation are the main reactions of free radicals in the absence of inhibitors. Excited molecules can lead directly to molecular products by dissociation or to higher products by dimerization reactions. Ionic species can yield molecular products by dissociation (if excited) or by ion-molecule reactions with the formation of a new ion in each case. Neutralization of the positive ions by electrons or by negative ions produces additional molecular products. 

The effect of electrical fields on the radiolysis of ethane has been examined by Ausloos et and this study has shown that excited molecules contribute a great deal to the products. The experiments were conducted in the presence of nitric oxide, and free-radical reactions were therefore suppressed. The importance of reactions (12)-(14) was clearly demonstrated by the use of various isotopic mixtures. Propane is formed exclusively by the insertion of CH2 into C2H6 and the yield is nearly equal to the yield of molecular methane from reaction (14). Acetylene is formed from a neutral excited ethane, probably via a hot ethylidene radical. Butene and a fraction of the propene arise from ion precursors while n-butane appears to be formed both by ionic reactions and by the combination of ethyl radicals. The decomposition of excited ethane to give methyl radicals, reaction (15), has been shown by Yang and Gant °° to be relatively unimportant. The importance of molecular hydrogen elimination has been shown in several studies ° °. ... 

It has been concluded that in the second-order kinetics of the decomposition of ammonia under silent electric discharge, the NH3 molecules play the role of sharing the de-excitation or neutralization energies of NH and... 

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