Electron beam absorption

There are some important limitations to TEM, 1) Electron beams are liable to produce high local temperature rises, and hence the metal cluster size and the location can be modified by the powerful electron beam, [146] 2) hundreds of metal clusters from many different micrographs should be measured in order to obtain a statistical representation of the size distribution, and 3) electron beam absorption by the zeolite crystal can hamper the detection of the particles. The best images seem to be those obtained from thin slabs of the zeolite crystals which have been cut at different heights and at different places with an ultramicrotome equipped with a diamond knife. The best images are from end-on views looking down into the pores with stacks of clusters in the cages. 

TEM Electron beam absorption/ scattering Bulk structures on morphology scale Phase contrast, cryo-microtoming Additionally contrasting, in vacuum... 

For a simplified case, one can obtain the rate of CL emission, =ft GI /e, where /is a function containing correction parameters of the CL detection system and that takes into account the fact that not all photons generated in the material are emitted due to optical absorption and internal reflection losses q is the radiative recombination efficiency (or internal quantum efficiency) /(, is the electron-beam current and is the electronic charge. This equation indicates that the rate of CL emission is proportional to q, and from the definition of the latter we conclude that in the observed CL intensity one cannot distii pish between radiative and nonradiative processes in a quantitative manner. One should also note that q depends on various factors, such as temperature, the presence of defects, and the... 

The classical approach for determining the structures of crystalline materials is through diflfiaction methods, i.e.. X-ray, neutron-beam, and electron-beam techniques. Difiiaction data can be analyzed to yield the spatial arrangement of all the atoms in the crystal lattice. EXAFS provides a different approach to the analysis of atomic structure, based not on the diffraction of X rays by an array of atoms but rather upon the absorption of X rays by individual atoms in such an array. Herein lie the capabilities and limitations of EXAFS. 

In 1951Castaing8 published results to show that an electron microscope could be converted into a useful x-ray emission spectrograph for point-to-point exploration on a micron scale. The conversion consisted mainly in adding a second electrostatic lens to obtain a narrower electron beam for the excitation of an x-ray spectrum, and adding an external spectrometer for analysis of the spectrum and measurement of analytical-line intensity. Outstanding features of the technique were the small size of sample (1 g cube, or thereabouts) and the absence of pronounced absorption and enhancement effects, which, of course, is characteristic of electron excitation (7.10). Castaing8 gives remarkable quantitative results for copper alloys the results in parentheses are the quotients... 

Fig. 11-1. Schematic diagram of x-ray absorption and emission analysis with a fine-focus x-ray tube. The focus is obtained by the magnetic lens acting on the electron beam. (Courtesy of Cosslett, Duncumb, Long, and Nixon, Proceedings, Sixth Annual Conference on Industrial Applications of X-ray Analysis, Denver, Colo., 1957, page 329.)...

Wavelength shift of K absorption edge of sulfur, 37, 38 Weight-fraction , use, 164, 171 Window absorption, 45 Windowless counter tube, 55, 222 Windows, for electron beam, 177 for proportional counter, 55... 

The polymers were dissolved in methylisobutylketone (MIBK) and spin-coated on oxjdized silicon wafers (1100 X thick Si02 layers) to form 5000 A thick films. After a prebaking to improve adhesion to the substrate, the resist samples were irradiated 0 through the mask (A) using the Al K 152 emission line at 8.3 A as X-ray source. The electron beam gun was operated at a 300 W power and the source to sample distance was U.9 cm. Taking into account the absorption of the aluminum foil mask,the different X-ray fluxes available on the sample were calculated from the relation given by (9) ... 

The verification of the presence of hydrogen in the film has proved more controversial, primarily because many of the structural investigations have been carried out after the film has been dried in vacuo. An example of the problems here is the fact that electron diffraction, which has to be carried out in vacuo, reveals a relatively well-crystallised spinel lattice whose origin may be the comparatively high sample heating encountered in the electron beam. Moreover, the use of in situ techniques, such as Mossbauer and X-ray absorption spectroscopy, clearly reveals marked differences between the spectra of the films in situ and the spectra of the same films ex situ as well as the spectra of y-Fe203 and y-FeOOH standards. These differences are most naturally ascribed to hydration of the spinel forms. 

In a study of long range ET between aromatic donor (biphenyl) and acceptor molecules separated by steroid spacers [39], pulse radiolysis and electron beam techniques have been used for the injection of electrons (Closs and Miller, 1988 Closs et al., 1989 Liang et al, 1990). Here, the reaction rates (observed by changes in the absorption spectra) pass through a... 

X = 0, CH2, CHCOOH, C(COOH)2, NH, NCH3 N(CH2CH=CH2), N(CHs)2 Cl Bobrowski and Das published a series of papers on the transients in the pulse radiolysis of retinyl polyenes31-37, due to their importance in a variety of biomolecular processes. They studied32 the kinetics and mechanisms of protonation reaction. The protons were released by pulse radiolysis, on a nanosecond time scale, of 2-propanol air-saturated solutions containing, in addition to the retinyl polyenes, also 0.5 M acetone and 0.2 M CCI4. Within less than 300 ns, the electron beam pulse results in formation of HC1. The protonated products of retinyl polyenes were found to absorb optically with Xmax at the range of 475-585 nm and were measured by this absorption. They found that the protonation rate constants of polyene s Schiff bases depend on the polyene chain... 

In bulk samples, X-ray yields need to be adjusted by the so-called "ZAF" correction. Z stands for the element number (heavier elements reduce the electron beam intensity more than lighter elements, because they are more efficient back-scatterers), A for absorption (different elements have different cross sections for X-ray absorption), and F for secondary fluorescence (the effect described above). Corrections are much less important when the sample is a film with a thickness of 1 pm or less, because secondary effects are largely reduced. The detection limit is set by the accuracy with which a signal can be distinguished from the bremsstrahlung background. In practice, this corresponds to about 100 ppm for elements heavier than Mg. 

To conduct such an experiment using electron beam radiation is more tedious, since the absorption cross section of organic materials to electrons is... 

The problems associated with quantitative studies of structure based upon this viewpoint of X ray absorption-edge spectra may be similar to those encountered using electron beams of comparable energy, 3 to 100 ev., to carry out electron diffraction studies of crystal structure. Qualitatively, this analogy can be carried further, as both the X ray spectra and the electron beam diffraction in this energy range are influenced by only the first few atom layers. 

Experimental details for the cross-section measurements were presented in the literature. Briefly, after the irradiation by electron beam pulse for a few nanoseconds, the time-dependent absorption for the atomic line transition Rg Rg -i-/zv was measured to observe the time-dependent population of the excited rare gas atoms Rg. The population of excited Rg was determined using an absorption law for the atomic lines, where the broadening of the absorption profile due to the thermal Doppler effect and due to the attractive interatomic potentials was reasonably taken into consideration. The time-dependent optical emission from energy transfer products, such as ... 

Pulse radiolysis was performed using e from a linear accelerator at Osaka University [42 8]. The e has an energy of 28 MeV, single-pulse width of 8 nsec, dose of 0.7 kGy, and a diameter of 0.4 cm. The probe beam for the transient absorption measurement was obtained from a 450-W Xe lamp, sent into the sample solution with a perpendicular intersection of the electron beam, and focused to a monochromator. The output of the monochromator was monitored by a photomultiplier tube (PMT). The signal from the PMT was recorded on a transient digitizer. The temperature of the sample solution was controlled by circulating thermostated aqueous ethanol around the quartz sample cell. Sample solution of M (5 x 10 -10 M) was prepared in a 1 x 1 cm rectangular Suprasil cell. 

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