Hemispherical analyzer

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... 

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). 

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... 

Hemispherical analyzer equipped with single channel or position sensitive detector offers excellent sensitivity and energy resolution. 

Most of the spectrometers are based on an electrostatic hemispherical analyzer, equipped with electrostatic lenses to collect, focus, retard or accelerate the photoelectron beam. 

An XPS spectrometer contains an X-ray source - usually Mg Ka (1253.6 eV) or A1 Ka (1486.3 eV) - and an analyzer which, in most commercial spectrometers, is hemispherical in design. In the entrance tube, the electrons are retarded or accelerated to a value called the pass energy , at which they travel through the hemispherical filter. The lower the pass energy, the smaller the number of electrons that reaches the detector, but the more precisely is their energy determined. Behind the energy filter is the actual detector, which consists of an electron multiplier or a channeltron, which amplifies the incoming photoelectrons to measurable currents. Advanced hemispherical analyzers contain up to five multipliers. For further details of these instruments the interested reader should refer to other textbooks [20, 21]. 

Figure 7.8 Working principles of a concentric hemispherical analyzer.

For time-resolved 2PPE spectroscopy, a combined set-up of an ultrafast laser system and an ultrahigh-vacuum photoemission spectroscopic system is indispensable. Typical electron energy analyzers have been used as the spectrometer, such as a cylindrical mirror analyzer, a hemispherical analyzer and a time-of-flight (TOF) analyzer. The TOF analyzer is mainly used for low repetition rate (<1 kFlz) laser sources, and the others are used for the lasers with multi-kldz or MHz repetition rates [11-14]. 

XPS analysis was performed on a Vacuum Generators (Fisons Instruments) MT-500 with a non-monochromatic A1 X-ray source (Ka 1486.6 keV) and a CLAM-2 hemispherical analyzer for electron detection. The samples were supported on carbon adhesive tape. Spectra were corrected for charging using the Si(2p) peak and scaled on the Si(2s) peak. For the determination of the binding energies a background correction was applied. 

Figure 1. Schematic of hemispherical analyzer showing electrons with low kinetic energy hitting the inner sphere, electrons with high kinetic energy hitting the outer sphere, and electrons with kinetic energy in the correct range to reach the exit slit and detector.

In our x-ray photoemission studies, a monochromatized photon beam (A1 Ka, hv = 1486.6 eV) was focused onto the sample surface, and the emitted electrons were energy analyzed with a Surface Sciences Instruments hemispherical analyzer. The take-off angle of the photoelectrons relative to the surface normal was 60° unless otherwise specified. A position-sensitive detector with 128 channels was used with a dedicated HP9836C computer to facilitate data acquisition. (22.) ... 

XPS-measuerements were carried out with an XPS equipment (SPECS) in ultra high vacuum (1 x 10 mbar) using MgKa radiation (1253.6 eV) and a hemispherical analyzer (PhoiboslOO) with an energy resolution of 1.2 eV. All spectra were normalised to the Au4f7/2-peak. 

Figure 17.3.5 Schematic diagram of X-ray photoelectron spectrometer with an electrostatic hemispherical analyzer. The detector is usually a channel electron multipher. [From J. J. Pireaux and R. Sporken in M. Grasserbauer and H. W. Werner, Eds., Analysis of Microelectronic Materials and Devices, Wiley, New York, 1991, with permission.]...

A thin layer of the catalyst was deposited upon the surface of the sample holder by sedimentation from a coarse-grained suspension of the sample in isopropyl alcohol. XPS measurements are performed in a UHV analysis chamber equipped with a multichannel detection hemispherical analyzer and a dual Al/Mg X-ray source operated at a power of 150 W. A preparation chamber enabling ion sputtering at high Ar current densities is also available. 

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. 

All experiments result in a spectriun which represents the energy distribution of photo electrons convoluted with instrumental functions from the analyzer and the detector system. These functions are fundamentally different for the two common analyzer types these are the cylindrical mirror analyzer (CMA) and the hemispherical analyzer (HSA). A description of the function of both devices can be found in the literature [2]. In commercial analytical instnunents, the HSA is more common than the CM A and all further discussion is limited to this device. 

The emitted photo electrons are collected by an electron optical device from a solid angle significantly wider than defined by the entrance aperture of the analyzer. Modern instruments use sophisticated lens systems allowing the imaging of the sample by the photoelectrons. The hemispherical analyzer is an electro-... 

Quantitative spectra with optimized resolution are recorded in the FAT mode with optimized pass energies. Figure 6 exemplifies the drastic effects of the analyzer convolution function on a photoemission spectriun for a valence band UPS experiment on graphite. The spectra characterize the very same surface and are both recorded with Hel radiation. The characteristic differences of the two operation modes of the hemispherical analyzer (LH EA 200) can be clearly seen as well as the different suitability of the relevant spectra for qualitative and quantitative analysis. In XPS data, the same differences occur for the two modes of operation. The intrinsically more symmetric line profile of core level lines tends, however, to obscure the effects of the analyzer in narrow scan spectra. 

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