Excited neutral molecule decomposition

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... 

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. 

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. 

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]. 

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

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. 

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... 

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... 

It therefore seems likely that C3H7OH2+ will be the major positive ion remaining in pure isopropyl alcohol after ion-molecule reactions, though it will be of the clustered form C3H7OH2+(C3H7OH)y, and that neutralization will proceed via Reaction 6. The mercury photosensitized decomposition of isopropyl alcohol shows a high quantum yield of H atoms (21) and a yield of H atoms from dissociation of excited isopropyl alcohol molecules might also be expected in the radiolysis. Hence, as for water vapor, the yield of H atoms from pure isopropyl alcohol will have... 

To explain the observed data, Groocock [231] assumed that electrons are excited to the conduction band, leaving holes in the valence band which become trapped at an impurity center decomposition occurs only when holes are trapped at neighboring impurity sites. Pai-Vernecker and Forsyth [119], on the other hand, assumed that photons create excited states of the azide ion, and the excitons become trapped on an impurity center decomposition occurs between the trapped excited molecule and a neighboring azide molecule in the ground state. In both cases it is assumed that electrons, freed from the azide anion, are responsible for neutralizing lead atoms which diffuse to form colloidal lead. 

The electron bombardment ionization of a molecule as described above produces an excited molecular ion, which, in attempting to gain stability, may decompose in a number of different ways by unimo-lecular reaction to fragment ions or neutrals. The relative abundance of the molecular ion is determined by its ability to resist decomposition, but the stability of fragment ions is dependent on the relative rates of reaction that form and destroy the ion. 

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