Concentric hemispherical

These modes of operation ate used in conjunction with the two most popular energy analyzers, the cylindrical mirror analyzer (CMA) and the concentric hemispherical analyzer (CHA). The most common form of the CMA used today is the double-pass version diagramed in Eigute 21. This device consists of two perfectly coaxial cylinders of radii r and r. The outer cylinder is held at a potential of (— ) and the inner cylinder is held at ground. The... 

The CHA is used for xps today in most commercial spectrometers. The most common version of the CHA is the 180° device shown in Figure 22. It consists of two concentric hemispherical surfaces of radii R2 and R. These surfaces have a potential difference of AH appHed between them so that the outer surface is negative and the inner surface is positive. The median equipotential surface Rq falls between these surfaces ideally, Rq = The... 

In electron spectroscopic techniques - among which XPS is the most important -analysis of the energies of electrons ejected from a surface is central. Nowadays universally employed is the concentric hemispherical analyzer (CHA). 

The energy of the photoelectrons leaving the sample are determined using a Concentric Hemispherical Analyser (CHA), and this gives a spectrum with a series of peaks whose energy values are characteristic of each element. A schematic diagram of a CHA is shown in Figure 2.3. 

Figure 2.3. Schematic diagram of a concentric hemispherical electron energy analyser.

Concentration standards, 75 750-751 Concentrators, sulfuric acid, 23 787 Concentric annular reactors, 23 544 Concentric cylinder viscometer, 27 733 Concentric hemispherical analyzer (CHA), 24 103-104, 105 energy resolution of, 24 106 Conching, milk chocolate, 6 363-364 Concomitant polymorphism, 8 69 CONCORD program, 6 10 76 752 Concrete(s)... 

Multichannel analyzer (MCA), 26 434 Multichannel detection, in concentric hemispherical analyzers, 24 106 Multiclient studies, 15 635-636 Multicollector-I CP-MS (MC-ICP-MS), archaeological materials, 5 743 Multicolor displays, LEDs in, 22 175 Multicompartment drum filters, 11 357 Multicomponent copolymerization, 7 619-620... 

In certain applications (e.g., X-ray photoelectron spectrometry) an electron beam must be analyzed by its kinetic energy using a 180° hemispherical energy analyzer consisting of two concentric hemispheres coupled to a wide-area charge-coupled detector (see Fig. 10.29). 

Figure 7.8 Working principles of a concentric hemispherical analyzer.

The CMA as shown is used for Auger electron spectroscopy. For XPS, two CMAs in series are used to obtain the required energy resolution. This design is called a double pass CMA. The transmission of electrons through a double pass CMA is good, but the resolution is poorer than that obtained using the concentric hemispherical analyzer described subsequently. 

Fig. 1. ScheiQatic cross section of a typical UHV system for multi-technique surface studies as employed in the authors laboratory (1) sample, (2) vibrating Kelvin probe, (3) quadrupole mass spectrometer, (4) LEED screen, (5) electron energy analyzer (electron lens system), (6) electron gun, (7) viewport, (8) concentric hemispherical analyzer, (9) electron detection system (electron multiplier). The system is also equipped with an ion sputter gun, a specimen manipulator and various gas inlet and pumping facilities.

Accordingly, 99% of the total resistance is within a sphere of radius 100a, and 90% within a sphere of radius 10a. The resistance value measured with a small spherical monopolar electrode may therefore show good spatial selectivity. A concentric hemisphere is an equipotential surface, and the surface can therefore be covered by a thin metal sheet acting as one electrode. It is not a neutral electrode until r is large. As a neutral electrode, a hemispherical electrode with r oo is equal to an infinite area bottom plate at infinite distance (Figure 6.2). 

If the incident radiation is monochromatic and of known energy, and if E can be measured using a high-resolution energy analyzer (such as either a concentric hemispherical or cylindrical mirror analyzer), then the binding energy E can be deduced. 

Figure 12.29 Diagram of a Concentric Hemispherical Analyzer (CHA) for XPS with a standard input lens system, which transfers an image of the analysed area on the sample onto the entrance slit to the analyzer, with slight magnification to permit removing the sample from close proximity to the entrance slit of the analyzer permitting greater working space around the sample. Source Reprinted with permission from Coxon P, Krizek J, Humpherson M, Warden IRM, Relat Phenomena, 52, 821, 1990. Copyright 1990, Elsevier.

There are many different designs of electron energy analyzers but the preferred option for photoemission experiments is a Concentric Hemispherical Analyzer (CHA), which uses an electric field between two hemispherical surfaces to disperse the electrons according to their kinetic energy (Fig. 5.9). 

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