Electron beam excitation

As mentioned earlier, CL is a powerful tool for the characterization of optical properties of wide band-gap materials, such as diamond, for which optical excitation sources are not readily available. In addition, electron-beam excitation of solids may produce much greater carrier generation rates than typical optical excitation. In such cases, CL microscopy and spectroscopy are valuable methods in identifying various impurities, defects, and their complexes, and in providing a powerful means for the analysis of their distribution, with spatial resolution on the order of 1 pm and less. ... 

Since the 1950s XRF has been used extensively for the analysis of solids, powders, and liquids. The technique was extended to analyze thin-film materials in the 1970s. XRF can be used routinely for the simultaneous determination of elemental composition and thickness of thin films. The technique is nondesuuctive, rapid, precise, and potentially very accurate. The results are in good agreement with other elemental analysis techniques including wet chemical, electron-beam excitation techniques, etc. 

In two other implementations of electron impact SNMS, a plasma is generated in the ionizer volume to provide an electron gas sufFiciendy dense and energetic for efficient postionization (Figure 2c). In one instrument, the electrons are a component of a low-pressure radiofrequency (RF) plasma in Ar, and in the second, the plasma is an electron beam excited plasma, also in Ar. The latter type of electron-gas SNMS is still in the developmental stages, while the former has been incorporated into commercial instmmentation. 

Excitation of sample by bombardment with electrons, radioactive particles or white X-rays. Dispersive crystal analysers dispersing radiation at angles dependent upon energy (wavelength), detection of radiation with gas ionization or scintillation counters. Non-dispersive semiconductor detectors used in conjunction with multichannel pulse height analysers. Electron beam excitation together with scanning electron microscopes. 

Non-destructive elemental analysis of solid or liquid samples for major and minor constituents. Used in routine analysis of metallurgical and mineral samples. Most suited to the determination of heavy elements in light matrices (e.g. Br or Pb in petroleum). Well suited for on-stream, routine analysis. Electron beam excitation methods valuable in surface studies in combination with electron microscopy. Detection limits generally in the range 10-100 ppm. Relative precision, 5-10%. 

Narrow bands at 320-335 nm with very short decay time of 20-30 ns may be confidently ascribed to Ce luminescence (Fig. 4.44). In steady-state spectra different bands in this spectral range without decay time analyses, especially under X-ray and electron beam excitations may be mistakenly considered as Ce " emissions (GOtze 2000). 

Fig. 3.1 Apparatus for. electron beam excitation of rare gas atoms (from ref. 9).

In scanning electron microscopy, the surface of a whole specimen is coated with a layer of heavy metal and then scanned with an electron beam. Excited molecules in the specimen release secondary electrons which are focused to produce a three-dimensional image of the specimen. 

Two distinct XeBr formation processes in electron-beam-excited Xe-Br2 mixtures have been identified in nanosecond pulse radiolysis experiments. One formation process, involving ion recombination, could be slowed relative to the second process, involving excitation energy transfer from Xe to Brz, by using very low electron-beam intensities, thereby allowing a kinetic analysis of the second process to be made. XeBr (B) emission at 280 nm has been observed following 193 nm excitation of IBr in Xe-IBr mixtures. ... 

X-Ray fluorescence is nondestructive and has significant advantages in simultaneous multielement analysis and ultramicroanalysis using electron beam excitation. It has found widespread industrial applications but as instrumentation is costly and complex in comparison with analytical atomic spectroscopy, the technique is not suitable for routine use in clinical chemistry. It seems unlikely that it can ever be more than a research tool. 

Figure 11.20 shows the influence of electron beam irradiation with =100keV in a transmission electron microscope on thin film structure during 2 s of deposition. As can be seen in Figure 11.20 the electron irradiation of the film results in its amorphization, as the main maximum at d= 0.435 nm attributing to the linear chain structure disappears the film structure transforms into diamond-like carbon. This means that the electron beam excitation of carbon atoms leads to cross-linkages among carbon chains and, as a result, the transformation of sp bonds into sp" and sp" bonds takes place. 

Various broadband sources employed to optically pump Ha include tungsten, mercury, xenon, and krypton lamps. The last source provides an especially good spectral match to the near-infrared absorption bands of Nd3+ in YAG. To reduce lattice heating resulting from the multiphonon emission decay to the F3/2 state, semiconductor diodes and laser sources at 0.8-0.9 ym nave pumped Nd lasers (58). Sun-pumped Nd and chromium-sensitized Nd lasers have been demonstrated and considered for space applications (59). Lasing of Nd3+ by electron beam excitation has also been reported (bO). 

An elegant molecular-beam study of the photofragmentation of aryl halides and methyl iodide has permitted extraction of excited-state lifetimes from a measured anisotropy parameter which depends upon the lifetime of excited state, the rotational correlation time of the molecule, and the orientation of the electronic transition dipole with respect to the —X bond.38 The lifetimes obtained were methyl iodide 0.07 ps, iodobenzene 0.5 ps, a-iodonaphthalene 0.9 ps, and 4-iodobiphenyl 0.6 ps, from which it was concluded that, whereas methyl iodide dissociates directly, the aryl halides predissociate. A crossed-beam experiment using electron-beam excitation has yielded the results for the Si Tt intersystem-crossing relaxation time in benzene, [sHe]benzene, fluorobenzene, and... 

A beam of electrons striking a target results in the emission of characteristic X-rays. This is the basis of the X-ray tube, as was discussed earlier in the chapter. A beam of electrons striking a sample will also generate characteristic X-rays from the sample. The use of a small diameter electron beam, on the order of 0.1 -1.0 p,m, to excite a sample is the basis of electron probe microanalysis. An electron probe microanalyzer is an X-ray emission spectrometer. The small diameter electron beam excites an area of the surface of the sample that is about 1 pm in diameter. Elemental composition and variation of composition on a microscopic scale can be obtained. 

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