Ion-impact reaction

Most of the electron and ion impact reactions that are included in this code are listed in Table II. Added to this are the elastic collisions SiHa -1- e [274] and H2 -f e [275], the recombination SiHJ + SiH 2SiH3 [214], and the charge exchange HJ -f H2 -1- H [193]. Note that disilane is not included. The... 

The existence of the dissociation glow in DC discharge strongly suggests that the creation of chemically reactive species in LCVD involves different mechanism than those in the electron impact ionization. However, in DC discharge, electron impact and ion impact reactions cannot be eliminated. Low-pressure cascade arc torch (LPCAT) provides a unique opportunity to investigate the formation of chemically reactive species with minimal influence of ions and electrons. That is, the creation of chemically reactive species from an organic molecule by the luminous... 

Ion-Impact Reaction. It has often been suggested that chemical change induced by electrical discharges is attributable to the positive ions present, but an analysis of the reaction kinetics appears to be lacking. For reactions analogous to Reaction Rl, and denoting a positive ion by I+ ... 

Method (b) corresponds to the usual method of investigating ion-molecule reactions in a high pressure mass spectrometer although charge exchange with slow ions is used instead of electron impact. After preliminary work (9, 23), the method was fully developed by Szabo 20, 21, 22). 

Some of the problems encountered in the mass spectrometric study of ion-molecule reactions are illustrated in a review of the H2-He system (25). If the spectrometer ion source is used as a reaction chamber, a mixture of H2 and He are subjected to electron impact ionization, and both H2+ and He+ are potential reactant ions. The initial problem is iden-... 

Reactions of Complex Ions. For reactions of systems containing H2 or HD the failure to observe an E 1/2 dependence of reaction cross-section was probably the result of the failure to include all products of ion-molecule reaction in the calculation of the experimental cross-sections. For reactions of complex molecule ions where electron impact ionization probably produces a distribution of vibrationally excited states, kinetic energy transfer can readily open channels which yield products obscured by primary ionization processes. In such cases an E n dependence of cross-section may be determined frequently n = 1 has been found. 

Since the observation made in study of the formation HeH+ indicated that this product was not formed by reaction of He + with H2, it had been assumed that the exothermic heat of reaction of He+ ions with H2 is probably deposited in the product HeH + as internal energy, decomposing the product into H+ and He. This idea was cited by Light (16) in his phase space theory of ion-molecule reactions to account for the failure to observe HeH+ from reactions with He+ ions. The experimental difficulty in the mass spectrometric investigation of this process is that H + formed by electron impact tends to obscure the ion-molecule-produced H+ so that a sensitive quantitative cross-section measurement is difficult. 

In 1960 Tal roze and Frankevich (39) first described a pulsed mode of operation of an internal ionization source which permits the study of ion-molecule reactions at energies approaching thermal energies. In this technique a short pulse of electrons is admitted to a field-free ion source to produce the reactant ions by electron impact. A known and variable time later, a second voltage pulse is applied to withdraw the ions from the ion source for mass analysis. In the interval between the two pulses the ions react under essentially thermal conditions, and from variation of the relevant ion currents with the reaction time the thermal rate constants can be estimated. 

In brief, the method consists of introducing small amounts (partial pressures of 10 3-10 4 torr) of the substance to be investigated into the ionization chamber of a mass spectrometer which contains a high pressure (1 torr) of methane, the reactant gas. Ionization is effected by electron impact, and because the methane is present in such an overwhelming preponderance, all but a negligibly small amount of the initial ionization occurs in the methane. The methane ions then undergo ion-molecule reactions to produce a set of ions which serve as reactant ions in the chemical ionization process. The important reactant ions formed from... 

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. 

FIG. 15. Electron impact reaction rales as a function of the average electron energy. A I 1 mixture of SiHa and H2 was used, at a total pressure of 83 Pa. (a) Reaction rates for SiHa, (b) reaction rates for Si2H6 (dotted lines) and H2 (solid lines). Abbreviations are ion. ionization dis, dissociation vib, vibrational excitation att, attachment. See Table II for details and references. (Adapted from G. J. Nienhuis, Ph.D. Thesis. Universileit Utrecht. Utrecht, the Netherlands. 1998.)... 

For the El ion source, the generated total ion stream is directly proportional to the gas pressure in the impact field, which provides a basic condition for quantitative analysis. Compounds can only safely be quantified if influences on the sensitivity of detection, such as ion-molecule reactions and competition in the ionisation process, can be excluded by experimental evidence. 

GC-IMR-MS is based on gentle ionisation of gas molecules by ion-molecule reactions (IMR) [242]. Such reactions between reaction ions and sample gases produce a significantly smaller excess of energy than does electron impact ionisation. Thus, IMR provides a... 

Rate Measurement. We have used pulsed ion cyclotron resonance (ICR) spectroscopy to study these gas-phase, ion-molecule reactions. The method has been described elsewhere in considerable detail (5). Basically, ions are generated by pulsed electron impact and held in a magnetic-electric field trap for times up to about 1 sec, during which they can react with a... 

Like in any catalytic process, process variables crucially impact reaction kinetics, conversion efficiency and catalyst stability. Increasing temperature favors cracking, thus decreasing the isomerate yield. It is preferred to have a high-activity catalyst and operate at the lowest possible temperature to achieve the highest RONC. Hydrogen shifts the equihbrium concentrations of olefins and carbenium ions. 

---