Electron bombardment

When subjected to an electron bombardment whose energy level is much higher than that of hydrocarbon covalent bonds (about 10 eV), a molecule of mass A/loses an electron and forms the molecular ion, the bonds break and produce an entirely new series of ions or fragments . Taken together, the fragments relative intensities constitute a constant for the molecule and can serve to identify it this is the basis of qualitative analysis. 

Figure Al.7.13. ESDIAD patterns showing the angular distributions of F emitted from PF adsorbed on Ru (0001) under electron bombardment, (a) 0.25 ML coverage, (b) the same surface following electron beam damage.

If the molecules could be detected with 100% efficiency, the fluxes quoted above would lead to impressive detected signal levels. The first generation of reactive scattering experiments concentrated on reactions of alkali atoms, since surface ionization on a hot-wire detector is extremely efficient. Such detectors have been superseded by the universal mass spectrometer detector. For electron-bombardment ionization, the rate of fonnation of the molecular ions can be written as... 

Lee Y T, McDonald J D, LeBreton P R and Herschbach D R 1969 Molecular beam reactive scattering apparatus with electron bombardment detector Rev. Sol. Instrum 40 1402-8... 

Fig. 19. Schematic of dual anode (typically Al and Mg) x-ray source. X-rays produced by electron bombardment of anode face 2 indicated (19). The routine dual-anode x-ray source just described generates nonmonochromatized x-rays in a relatively large spot size (ca 1 cm in diameter). In...

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... 

Chain reactions such as those described above, in which atomic species or radicals play a rate-determining part in a series of sequential reactions, are nearly always present in processes for the preparation of thin films by die decomposition of gaseous molecules. This may be achieved by thermal dissociation, by radiation decomposition (photochemical decomposition), or by electron bombardment, either by beams of elecuons or in plasmas. The molecules involved cover a wide range from simple diatomic molecules which dissociate to atoms, to organometallic species with complex dissociation patterns. The... 

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... 

Yes caution—in certain cases electron bombardment may ionize or create defects... 

The number of Auger electrons from a particular element emitted from a volume of material under electron bombardment is proportional to the number of atoms of that element in the volume. However it is seldom possible to make a basic, first principles calculation of the concentration of a particular species from an Auger spectrum. Instead, sensitivity factors are used to account for the unknown parameters in the measurement and applied to the signals of all of the species present which are then summed and each divided by the total to calculate the relative atomic percentages present. 

Other considerations are that the source material, which forms a target for high-energy electron bombardment leading to the production of X-rays, should be a good conductor- to enable rapid removal of heat - and should also be compatible with UHV. 

For efficient production of X-rays by electron bombardment, exciting electron energies that are at least an order of magnitude higher than the line energies must be used, so that in Mg and A1 sources accelerating potentials of 15 kV are employed. Mod-... 

The progressive ordering from the surface to the center has been experimentally observed in the case of the electron irradiation-induced formation of the quasi-spherical onion-like particles[25]. In this case, the large inner hollow space is unstable under electron bombardment, and a compact particle (innermost shell C( ) is the final result of the graphitization of the carbon volume (see Fig. 3e-h). 

Unfortunately, not every compound shows a molecular ion in its mass spectrum. Although M+ is usually easy to identify if it s abundant, some compounds, such as 2,2-dimelhylpropane, fragment so easily that no molecular ion is observed (Figure 12.3). In such cases, alternative "soft" ionization methods that do not use electron bombardment can prevent or minimize fragmentation. 

Hydroxylamine, NHjOH, is subjected to electron bombardment The products are passed through a mass spectrograph. The two pairs of lines formed indicate charge/mass ratios of 0.0625,0.0588 and 0.1250,0.1176. How can this be interpreted ... 

The short-wavelength limit of the continuous spectrum is clearly a quantum phenomenon. X-ray generation by electron bombardment in principle resembles cathodoluminescence, and both processes are inverse photoelectric effects. The short-wavelength limit, Xq, discovered by Duane and Hunt6 obeys the relationship... 

Fig. 4-1. Calculated intensity-wavelength distributions for continuous spectra produced by electron bombardment of various targets. The ordinate scale is arbitrary.

The discussion so far applies alsr> to the excitation of characteristic x-rays by electron bombardment of a sample. In such excitation, the continuous spectrum will always appear as an appreciable and often objectionable background. The discussion also bears on the excitation of x-rays by x-rays to the extent that it describes the kind of x-ray beam often used for excitation. 

The discussion just concluded is largely implicit in the earlier discussion of the excitation of a continuous x-ray spectrum by electron bombardment (4.1). Note that x-rays behave differently when they are used for excitation. An x-ray penetrates with little or no loss of energj" until it is absorbed, and it is the more likely to penetrate to greater depth (in regions of continuous abiorption) the greater its energy (or shorter its wavelength). 

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