Electron beam measurements

Fig. 2.4 Low-energy electron diffraction (LEED). (a) Apparatus, showing how electrons reflected from a surface are detected by a fluorescent screen, (b) LEED pattern obtained from the surface of a tungsten oxide crystal. The bright spots show reflected electron beams. Measurement of their angles and Intensities gives information about the positions of atoms on the surface.

Fig. 16.1. Experimental setup. The focal spot X-ray image is also shown as an inset, as well as the well-collimated fast electron beams measured to have energies greater than 250 keV arising from ultrashort laser interactions with the plasma at...

The intensity of the electron beam, measured on Electrode 8 in the absence of gas (F — 10"6 mm. Hg) was fixed at 50 pamp. for all energies used. Below the threshold energy for radical production, this intensity remains practically constant when ammonia is admitted in the ion source. At higher energies its value depends, of course, on the reactions involving ions and electrons which occur in the source. 

The (details of these measurements are beyond the scope of this text Because they require an ionizing electron beam, measured proton afhnities d gas-[ ase basicities fcx many species have large uncertainties, because the molecules involved frequently are in excited states (with excess energy above their ground states), and some species do not yield the necessary acid as a gaseous fragment Relatively few molecules are ideally suited f[Pg.177]

Figure Al.7.12. Secondary electron kinetic energy distribution, obtained by measuring the scadered electrons produced by bombardment of Al(lOO) with a 170 eV electron beam. The spectrum shows the elastic peak, loss features due to the excitation of plasmons, a signal due to the emission of Al LMM Auger electrons and the inelastic tail. The exact position of the cutoff at 0 eV depends on die surface work fimction.

In a very simple fonn of electron spectroscopy, known as electron transmission spectroscopy, the attenuation of an essentially monoenergetic beam of electrons is measured after passage tlnough a sample. If the target is very thin or of such low density that most electrons pass tlnough unscattered, the attenuation is small and the... 

Except for the phase-contrast detector in STEM [9], STEM and SEM detectors do not track the position of the recorded electron. The spatial information of an image is fonned instead by assigning the measured electron current to the known position of the scaimed incident electron beam. This infomiation is then mapped into a 2D pixel array, which is depicted either on a TV screen or digitalized in a computer. 

A low-energy electron beam can also be obtained using a field emission tip and used in the field emission retarding-potential method. This combination provides an absolute measure of the sample work fiinction and the resolution is excellent [52]. 

If monochromatic X-rays are used as the ionizing radiation the experimental technique is very similar to that for XPS (Section 8.1.1) except that it is the kinetic energy of the Auger electrons which is to be measured. Alternatively, a monochromatic electron beam may be used to eject an electron. The energy E of an electron in such a beam is given by... 

The unknown and standards must be measured under identical conditions of beam energy and spectrometer parameters. The specimen s surface must be oriented to known angles relative to the electron beam and the detector. All meas-... 

The simplest diffraction measurement is the determination of the surface or overlayer unit mesh size and shape. This can be performed by inspection of the diffraction pattern at any energy of the incident beam (see Figure 4). The determination is simplest if the electron beam is incident normal to the surface, because the symmetry of the pattern is then preserved. The diffraction pattern determines only the size and shape of the unit mesh. The positions of atoms in the surface cannot be determined from visual inspection of the diffraction pattern, but must be obtained from an analysis of the intensities of the diffracted beams. Generally, the intensity in a diffracted beam is measured as a fimction of the incident-beam energy at several diffraction geometries. These intensity-versus-energy curves are then compared to model calculations. ... 

Surface atomic structure. The integrated intensity of several diffracted beams is measured as a fimction of electron beam energy for different angles of incidence. The measurements are fitted with a model calculation that includes multiple scattering. The atomic coordinates of the surfiice atoms are extracted. (See also the article on EXAFS.)... 

The complete description of the number of Auger electrons that are detected in the energy distribution of electrons coming from a surface under bombardment by a primary electron beam contains many factors. They can be separated into contributions from four basic processes, the creation of inner shell vacancies in atoms of the sample, the emission of electrons as a result of Auger processes resulting from these inner shell vacancies, the transport of those electrons out of the sample, and the detection and measurement of the energy distribution of the electrons coming from the sample. 

AES analysis is done in one of four modes of analysis. The simplest, most direct, and most often used mode of operation of an Auger spectrometer is the point analysis mode, in which the primary electron beam is positioned on the area of interest on the sample and an Auger survey spectrum is taken. The next most often used mode of analysis is the depth profiling mode. The additional feature in this mode is that an ion beam is directed onto the same area that is being Auger analyzed. The ion beam sputters material off the surface so that the analysis measures the variation, in depth, of the composition of the new surfaces, which are being continu-... 

It is a fundamental principle of quantum mechanics that electrons bound in an atom can have only discrete energy values. Thus, when an electron strikes an atom its electrons can absorb energy from the incident electron in specific, discrete amounts. As a result the scattered incident electron can lose energy only in specific amounts. In EELS an incident electron beam of energy Eq bombards an atom or collection of atoms. After the interaction the energy loss E of the scattered electron beam is measured. Since the electronic energy states of different elements, and of a single element in different chemical environments, are unique, the emitted beam will contain information about the composition and chemistry of the specimen. 

In addition to qualitative analysis of nearly all the elements of the periodic table, EEL spectra also enable determination of the concentration of a single element which is part of the transmitted volume and hence gives rise to a corresponding ionization edge. As in all comparable spectroscopic techniques, for quantification the net edge signal, which is related to the number N of excited atoms, must be extracted from the raw data measured. The net intensity 4 of the feth ionization shell of an individual element is directly connected to this number, N, multiplied by the partial cross-section of ionization ) and the intensity Iq of the incident electron beam, i.e. ... 

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