Gases, electron bombardment

Unstable monohaUdes of xenon ([16757-14-5], XeF [55130-03-5], XeCl [55130-04-6], XeBr and [55130-05-7], Xel), have been produced in the gas phase by electron bombardment methods (43,44) and in soHd matrices by gamma and ultraviolet inradiation methods (45,46). Although short-Hved in the gas phase, these haUdes are of considerable importance as light-emitting species in gas lasers (qv). 

Diels-Alder reactions, 4, 842 flash vapour phase pyrolysis, 4, 846 reactions with 6-dimethylaminofuKenov, 4, 844 reactions with JV,n-diphenylnitrone, 4, 841 reactions with mesitonitrile oxide, 4, 841 structure, 4, 715, 725 synthesis, 4, 725, 767-769, 930 theoretical methods, 4, 3 tricarbonyl iron complexes, 4, 847 dipole moments, 4, 716 n-directing effect, 4, 44 2,5-disubstituted synthesis, 4, 116-117 from l,3-dithiolylium-4-olates, 6, 826 electrocyclization, 4, 748-750 electron bombardment, 4, 739 electronic deformation, 4, 722-723 electronic structure, 4, 715 electrophilic substitution, 4, 43, 44, 717-719, 751 directing effects, 4, 752-753 fluorescence spectra, 4, 735-736 fluorinated derivatives, 4, 679 H NMR, 4, 731 Friedel-Crafts acylation, 4, 777 with fused six-membered heterocyclic rings, 4, 973-1036 fused small rings structure, 4, 720-721 gas phase UV spectrum, 4, 734 H NMR, 4, 7, 728-731, 939 solvent effects, 4, 730 substituent constants, 4, 731 halo... 

The use of inadiation or electron bombardment offers an alternative approach to molecular dissociation to the use of elevated temperamres, and offers a number of practical advantages. Intensive sources of radiation in the visible and near-visible are produced by flash photolysis, in which a bank of electrical capacitors is discharged tlrrough an inert gas such as ktypton to produce up to 10 joule for a period of about 10 " s, or by the use of high power laser beams (Eastham, 1986 (loc.cit.)). A more sustainable source of radiation is obtained from electrical discharge devices usually incorporating... 

I.I. Electron Bombardment Plasma Sources. These gas-feed sources generally employ Ar or Xe at relatively low vapour pressures (10 3mbar). A heated cathode is a common electron source, and these are accelerated towards an anode to give them... 

The data from those methods that involve electron bombardment or emission will suffer from the scattering of the electrons if any gas molecules are present. 

A mass spectrometer is often indispensable for a complete analysis of low-pressure gases, but a description of the various types of spectrometers is beyond the purpose of this book, but see, for example, ref. [18]. We simply remind that a mass spectrometer consists of three parts an ion source where the neutral gas is ionized (usually by electron bombardment) an analyser where ions are selected according to their mass to charge ratio and a collector with an amplifier to measure the weak ion current. 

Chemical ionization (Cl) sources (48, 49) use electron bombardment of a reagent gas at higher pressures than normally found in a mass spectrometer ion source, i.e., torr. Sample ionization follows via an ion-molecule reaction, often accompanied by a proton transfer to yield a quasi-molecular ion ... 

An instrument that measures the isotopic mass ratio of a gas by bombarding the sample in an electron beam, such that the molecular ions generated can be deflected in their trajectories through a magnetic field in accordance to their charge/mass ratios. These devices are extremely accurate and reliable, and many stable isotope experiments can be analyzed by converting the isotopi-caUy substituted metabolite into carbon dioxide, water, or molecular nitrogen prior to I RMS measurements. 

The interaction of an electron with a surface produces at least three phenomena which are important in a plasma environment. They are (1) chemical reactions between gas phase species and a surface where electron bombardment is required to activate the process, (2) electron-induced secondary-electron emission, and (3) electron-induced dissociation of sorbed molecules. A fourth phenomenon — lattice damage produced by energetic electrons — depends sensitively upon the properties of the material being bombarded, and, it is important in specialized situations, but it will not be discussed in this paper. 

It is the suspicion of the present author that gas-solid chemical reactions, which happen only in the presence of ion or electron bombardment, are a widely occurring phenomena in a plasma environment. Nevertheless, there are few if any well defined investigations of this topic. This is a consequence of the fact that most reactions of this type (e.g., oxidation or nitridization) produce a thin layer of non-volatile reaction product on the surface and then the reaction stops. A chemical reaction of this type is very difficult to investigate space experimentally. However, when the product is volatile, then the reaction is much easier to study since it continues indefinitely. Therefore, electron-induced chemical reactions will be discussed using an example where the reaction product is volatile. 

Exposure of Si02, Si3N4, or SiC to XeFiCgas) produces an adsorbed layer of fluorine . The xenon does not remain on the surface, but it is immediately desorbed into the gas phase. This is all that happens in the absence of electron bombardment. However, in the presence of electron bombardment, SiF4(gas) and other volatile products are produced . Since the reaction products are removed from the surface, the reaction proceeds until all the material is volatilized. This is illustrated for the case of SiOj in Fig. 32. Similar data has been obtained for SijN4 which reacts faster and for SiC which reacts slower. These are examples of a class of chemical reactions which require both active species (in this case fluorine) and energetic radiation (in this case electrons). 

Many of the mechanisms discussed in Sect. 2.2.6.1 with regard to ions may also apply to chemical reactions enhanced by electron bombardment. A discussion of that type will not be repeated in this section. However, a mechanism for the electron-enhanced etching of SiOj can be suggested on the basis of processes which are known to occur. It is known, for example, that electron bombardment of SiOj causes oxygen to be desorbed into the gas phase, i.e., electron stimulated desorption occurs . The silicon which remains upon the surface can now be attacked by the XeFjfgas) to produce SiF4(gas). In this manner both oxygen and silicon are removed from the SiOj lattice and the material is etched. The chemistry involved is likely to be more complex, but this simple model illustrates a possible mechanism. 

Electron bombardment of gas mixtures, although not strictly a photochemical process, has been used in conjunction with time-resolved SS interferometers to obtain rate constants for the vibrational relaxation of highly excited molecules [30, 35, 36, 71-73], Murphy et al. [35,71] observed the production and relaxation of vibrationally excited NO and the (0,0,1 - 0,0,0) bands of N20 and N02 following excitation of N2/02 mixtures with a pulsed electron gun. The infrared emission created by the electron beam decayed completely between pulses and the complete temporal... 

Production of Ions. Several methods are used (11 by bombardment with electrons from a heated filament (2 by application of a strong electrostatic field (field ionization, field desorption) Ot by reaction with an ionized reagent gas (chemical ionization) (4 by direct emission of ions from a solid sample that is deposited on a heated filament (surface ionization) (SI by vaporization from a crucible and subsequent electron bombardment (e.g.. Knudsen cell for high-lcmperalure sludies id solids and (6) by radio-frequency spark bomhardmenl of sample fur parts-per-biliion (ppb) elemental analysis of solids as encountered in metallurgical, semiconductor, ceramics, and geological studies. Ions also are produced by photoion izution and laser ionizalion. 

Most of the high precision spectroscopy of He Rydberg states has been done by microwave resonance, which is probably the best way of obtaining the zero field energies. Wing et a/.8-12 used a 30-1000pA/cm2 electron beam to bombard He gas at 10-5-10-2 Torr. As electron bombardment favors the production of low states, it is possible to detect A transitions driven by microwaves. The microwave power was square wave modulated at 40 Hz, and the optical emission from a specific Rydberg state was monitored. When microwave transitions occurred to or... 

The principal impediment to effective process design and analysis is the limited understanding of synergistic effects due to ion, photon, and electron bombardment of solid surfaces during etching and deposition. Fundamental relationships must be established between the gas-phase chemistry the surface chemistry as modified by radiation and etch profiles, rates, selec-tivities, and film properties. 

The decompn of various azides by electron bombardment has been studied by a number of investigators. Muller and Brous (Refs 5 8) studied Na azide, and Groocock and Tompkins (Ref 38) also investigated Ba azide by this technique. Groocock (Ref 80) continued his expts with a-Pb azide and produced a curve similar to the ones for Ba and Na azides. In all of these studies only gas evolution was studied. An interesting feature observed by Groocock was a decompn mechanism varying with the thickness of the azide layer which implied that... 

Early Unsuccessful Attempts. Until the early 1960s, simple alkyl cations were considered only as transient species.15 Their existence has been inferred from the kinetic and stereochemical studies of reactions. No reliable physical measurements, other than electron impact measurements in the gas phase (mass spectrometry), were known. The formation of gaseous organic cations under electron bombardment of alkenes, haloalkanes, and other precursors has been widely investigated in mass spectrometric studies.81 No direct observation of carbocations in solutions was achieved prior to the early 1960s. 

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