Electrostatic lens, electrons

Compare a TEM to a tremsmitted light optical microscope being used to take a photomicrograph, and the principles of design are very similar. Illumination of the object is very important, and in both microscopes the resolution and contrast of the image may be degraded if the illumination is not properly adjusted. The source of illumination may be a small, hot tungsten filament in both cases. The electrons or photons emitted from the filament are collected by a condenser lens. To increase efficiency in the TEM, an electrostatic lens electron gun) is used to steer more of the flux into this lens. In the optical microscope, a mirror behind the lamp performs this function. A second condenser lens controls the transfer of this illumination to the specimen plane. In the TEM, these are simply called condenser lens 1 (Ci) and condenser... 

It is easiest to discuss the electron optics of a TEM instrument by addressing the instrument from top to bottom. Refer again to the schematic in F ure la. At the top of the TEM column is an electron source or gun. An electrostatic lens is used to accelerate electrons emitted by the filament to a h potential (typically 100-1,000 kV) and to focus the electrons to a cross-over just above the anode (the diameter of the cross-over image can be from 0.5 to 30 Mm, depending on the type of gun ). The electrons at the cross-over image of the filament are delivered to the specimen by the next set of lenses on the column, the condensers. 

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... 

Figure 6. Diagram of our 1-atm ion mobility spectrometer (IMS) apparatus (a) stainless steel source gas dilution volume, (b) septum inlet, (c) needle valve, (d) Nj source gas supply, (e) source and drift gas exhaust, (f) flow meter, (g) pressure transducer, (h) insulated box, (i) drift tube, (j) ion source, (k) Bradbury-Nielson gate, (I) Faraday plate/MS aperture, (m) drift gas inlet, (n) universal joint, (o) electrostatic lens element, (p) quadrupole mass filter, (q) 6"-diffusion pump, (r) first vacuum envelope, (s) channeltron electron multiplier, (t) second vacuum envelope, (u) 3"-dif-fusion pump, (v) Nj drift gas, (w) leak valve, (x) on/off valves, (y) fused silica capillary, (z) 4-liter stainless steel dilution volume, (aa) Nj gas supply.

The data presented here were taken with an apparatus similar to the one recently described by Parker and Eppink [25], The difference between it and the original photofragment imaging apparatus of Chandler and Houston [23] is that the flat screens, through which the ions or electrons are projected were replaced by a series of concentric electrodes. These form an electrostatic lens that, when the appropriate voltages are applied, will focus... 

The passage of electrons or other particles with charge q and mass m through an electrostatic lens system is governed by their motion under the action of the electric field. In the case considered here, cylindrical symmetry around the optical axis (z-axis) and paraxial rays will be assumed. Of the cylindrical coordinates only the transverse radial coordinate p and the distance coordinate z are of relevance, and the electrostatic potential of the lens is given by q>(p, z). As shown in Section 10.3.1, in the paraxial approximation the potential q>(p, z) is fully determined by the potential symmetry axis. Hence, the equations of motion and the fundamental differential equation of an electrostatic lens depend only on this potential. The fundamental lens equation is given by (see equ. (10.38))... 

Syms RRA, Tate TJ, Ahmad MM, Taylor S (1998) Design of a microengineered quadrupole electrostatic lens. IEEE Trans Electron Dev TED 45 2304-2311... 

The ion beam is produced in the following way U atoms are evaporated from an oven at a temperature of typically 400 C, and are ionized and excited to the metastable 2 5 state by electron impact, when leaving the oven aperture. The electrons are emitted from a little ring-shaped tungsten wire cathode which is placed horizontally several millimeters above the oven exit. The cathode is held at ground potential, the oven at +200 V. The electrons are accelerated directly onto the oven aperture thus counterpropa-gating to the ions which are accelerated in the same electric field. The ions pass the cathode loop and are formed into a well-collimated beam by an electrostatic lens system. 

Figure 5.100 shows a simpMed diagram of an electrostatic lens. An electron emitted at zero velocity enters the Vi region and moves in that region at a constant velocity (because the region has a constant potential). The velocity of the electron in that region is defined by Eq. (5.10) for the straightiine component. 

Electrons ejected from the sample are collected by an electrostatic lens (h in Fig. 7), and they are sorted out according to their kinetic energy in an energy analyzer (i in Fig. 7), as schematized in Fig. 2. 

We disccunt the electrostatic lens effect occurring in the acceleration zone of the thruster channel, causing the divergence of the jet. The ionization reaction is Xe + e Xe + e + e. The energy of the electrons creates few doubly-charged ions. 

Figure 3.2.2.S Cross-sectional view of the Scienta R4000 hemispherical electrostatic energy analyzer equipped with a two-dimensional detector for parallel detection in energy and emission angle. Typical electron trajectories are shown for two different energies, starting at the sample surface and indicating both the focusing by the electrostatic lens and the hemispherical analyzer, as well as the energy dispersion on the detector plane. A similar dispersion takes place...

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