Electrostatic focusing lenses

Electron impact (El) ion sources are the simplest type. O2, Ar, or another (most often noble) gas flows through an ionization region similar to that depicted in Eig. 3.30. Electrons from an incandescent filament are accelerated to several tens of eV by means of a grid anode. A 20-100 eV electron impact on a gas atom or molecule typically effects its ionization. An extraction cathode accelerates the ions towards electrostatic focusing lenses and scanning electrodes. 

The purpose of the ion source, as the term implies, is to provide the energy necessary to ionize the analyte molecules, while being maintained at a temperature high enough to prevent analyte condensation. In addition, electrostatic focusing lenses are usually included to accelerate the ions and collimate the ion beam. The two types of ionization normally used in GCMS are electron ionization (El) and chemical ionization (Cl). The specifics of their operation are covered in Sections 7.3 and 7.4, respectively. 

Figure Bl.7.4. Schematic diagram of a reverse geometry (BE) magnetic sector mass spectrometer ion source (1) focusing lens (2) magnetic sector (3) field-free region (4) beam resolving slits (5) electrostatic sector (6) electron multiplier detector (7). Second field-free region components collision cells (8) and beam deflection electrodes (9).

MicroChannel Electrostatic focusing piate Collector and acceleration lens ... 

The ion source with the extraction system is connected to the accelerator or directly to the target through a transport system. The transport system consists of several elements. After the extraction optics, an active beam-focusing tool is usually applied (electrostatic einzel lens or solenoid) followed by a charge separator. Diagnostics units are installed in the beam line to measure the beam quality. The most important beam parameters are the ion current, beam profile, and emittance. The beam current is measured by a special cup-shaped metal electrodes (Faraday cups) that convert the intensity of the stopped ion beam into electrical current. The beam profile and emittance can be diagnosed by several methods. The detailed description of different transport systems can be found in the ion source handbooks (Wolf 1995 Brown 2004). 

The magnification can be controlled by changing this ratio, which in turn changes the size of the spot. This is one way to control the quahty of the focus. Although the actual lens may not be thin, and in general is more comphcated than is shown in the figure, the illustration is sufficient to understand the operation of electrostatic focus. 

Once the ions have been successfully extracted from the interface region, they are directed into the main vacuum chamber by a series of electrostatic lens, called ion optics. The operating vacuum in this region is maintained at about 10" torr with a turbomolecular pump. There are many different designs of the ion optic region, but they serve the same function, which is to electrostatically focus the ion beam toward the mass separation device, while stopping photons, particulates, and neutral species from reaching the detector. 

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. 

A simple spectrometer that we have used successfully is shown in Figure 2. Electrons from an electron microscope hairpin tungsten filament are focused with an Einzel lens onto the monochromator entrance slit, pass through the monochromator and exit slit, and are focused on the sample s surface by additional electrostatic... 

The difficulty, of course, was that electrons cannot be focused by a glass lens, and it was necessary to use either magnetic or electrostatic Menses . A German, Hans Busch, in 1926/27 published some seminal papers on the analogy between the effect... 

Fig. 3.11 Poschenrieder ion flight time focusing scheme which uses a 163° spherical electrostatic lens.

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... 

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