Analyzer electrostatic mirrors

Time Focusing Devices. The resolution of the TOF analyzer is limited by the initial velocity spread of the ions. However, there are powerful devices that can compensate for this velocity distribution, and the most widespread techniques at present are the electrostatic ion reflector (electrostatic mirror) and time-lag focusing (delayed extraction). 

Figure 9.8 Schematic of a reflectron time of flight mass analyser. Reflectron lenses act as an electrostatic mirror to both increase the effective length of the flight path, but also to compensate for ion kinetic energy variations (Uq), resulting in higher mass accuracy relative to purely linear time of flight mass analyzers. Consequently, linear time of flight analyzers are nowadays largely obsolete.

The TOF is the most widely used analyzer for SIMS experiments. The TOF is based on the measurement of the time elapsed between the impact of the pulsed primary beam on the sample surface and the detection of the emitted secondary ions by the ion detector. The flight time of ions with different m/z (typically larger than 1 ps) is proportional to the ratio itself and is used to analyze the different ions. TOF analyzers are often used in conjunction with pulsed primary ion sources because the latter offer the possibility to synchronize the ion detection with the primary ion source pulse frequency. Reflectron TOF analyzers compensate for the secondary ion kinetic energy dispersion by using an electrostatic mirror that gradually reflects ions with the same m/z but with different kinetic energy. 

In the configuration presented previously, the TOF analyzer presents the inconvenience of low resolution. Due to turbulence in the acceleration zone, collisions with neutrals. Coulomb repulsions, and other factors, ions of the same m/z ratios acquire slightly different kinetic energies and reach different speeds and times of flight. This translates to an increase in dispersion of times of flight and a decrease in resolution. Several attempts have been made to solve this problem. Delayed extraction achieved some success but the electrostatic mirror surpasses the other inventions. ... 

Figure 3 demonstrates the electron spectrometer part of a depth-resolved conversion electron MOssbauer spectrometer specially designed for such measurements in our laboratory (10, 11). The electron spectrometer is of the cylindrical mirror type back-scattered K conversion electrons from resonantly excited Fe nuclei are resolved by the electrostatic field between the inner and outer cylinders (cylindrical mirror analyzer) and then detected by a ceramic semiconductor detector (ceratron). The electron energy spectra taken with this spectrometer indicate that peaks of 7.3-keV K conversion electrons, 6.3-keV KLM Auger electrons, 5.6-keV KLL Auger electrons, etc., can be resolved well, with energy resolution better than 4%. 

In electron spectroscopic techniques—among which XPS is the most important—analysis of the energies of electrons ejected from a surface is central. Because of the low kinetic energies involved in the techniques, analyzers using magnetic fields are undesirable. Therefore the energy analyzers used are exclusively of the electrostatic deflection type. The two that are now universally employed are the concentric hemispherical analyzer (CHA) and the cylindrical mirror analyzer (CMA). Since both have been used in XPS, both are described here, although in practice the CHA is more suitable for XPS, and the CMA for AES. 

An alternative design is based on electrostatic beam transport and focussing. Figure 3.4 shows schematically the Aarhus electrostatic positron beam line (Deutch et al. [3.9]). A 70mCi 8 mm diameter Na source delivers particles that are moderated in a laser-annealed W(100) foil. The transmitted slow positrons are collected and accelerated by a modified Soa immersion lens (Canter et al. [3.10]). They are then further accelerated and deflected through 90° in a cylindrical mirror analyzer and thus removed from the primary P" and 7 background of the source. The beam is then transported and focussed to a spot of 2 mm diameter on the target, where the positron intensity is 10 s ... 

Fig. 7 X-ray photoelectron spectrometer. Left schematic view of a SSX 100/206 (Surface Science Instruments). Right, photographs of a Kratos Axis Ultra (Kratos Analytical) with the introduction and intermediate chambers (top) and analysis chamber (bottom), a, Turbomolecular pump b, cryogenic pump c, introduction chamber d, sample analysis chamber (SAC) e, transfer probe f, automatized X, Y, Z manipulator g, X-ray monochromator h, electrostatic lens i, hemispherical analyzer (HSA) j, ion gun k, aluminum anode (with monochromator) 1, aluminum-magnesium twin anode m, detector. Left channel plate. Right 8 channeltrons (Spectroscopy mode), phosphor screen behind a channel plate with a video camera (Imaging mode) n, spherical mirror analyzer (SMA) o, parking facility in the sample transfer chamber p, sample cooling device for the introduction chamber q, sample transfer chamber r, monitor interconnected with the video camera viewing samples in the SAC s, video camera in the SAC t, high temperature gas ceU (catalyst pretreatment)...

Fig. 17. Schematic representation of a reflectron (ion mirror) in a TOF analyzer Ions with the same m/z formed with higher initial velocity (--) penetrate deeper in the electrostatic field of the reflec-...

Most commercial ISS equipment only analyzes for charged particles, and particles that are neutralized on reflection are lost. The energy of the scattered ion is typically analyzed by an electrostatic sector analyzer or a cylindrical mirror analyzer. Ions for bombardment are provided by an ion source. Depth profiling can be done using sputter profiling techniques. 

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