Electron sources and analysers

One big advantage of using electrons is that they are relatively easy to produce. The most common way is electron emission from a hot filament. A filament is heated by passing a current through it. To ``help'' the thermally excited electrons out of the metal one additionally puts an anode in front of the filament. The electron beam is focused by placing a so-called Wehnelt cylinder between the anode and the filament. The Wehnelt cylinder is at a negative potential with respect to the filament. The basic principle is shown in Fig . The simple filament has two disadvantages when one eventually wants to produce a monochromatic beam of electrons. The first is that the voltage drop over the length of the filament (0.5 V) is also reflected in the kinetic energy of the electrons. The second is the thermal broadening due to the high temperature needed to emit the electrons. A better design for emitting monochromatic electrons is an indirectly heated crystal which has a low work function.

  
Figure: An electron gun.

Electrons can be detected using an electron multiplier, usually a so-called channeltron  . Such a device is essentially a glass tub with a resistive coating on the inside. A high voltage is applied between the front and the end. An electron which enters the channeltron will be accelerated to the wall where it kicks out more electrons. In this way an electron avalanche is created which eventually leads to a measurable current pulse. Electron monochromators are needed both for creating a mono-energetic probe-beam and for analysing the energy distribution of scattered or emitted electrons. Electrostatic monochromators are the most common choice. Actual designs represent a trade-off between the need for high count rates and high angular / energy resolution. The so-called cylindrical mirror analyser (CMA)   is mostly used for checking the chemical composition of the surface. It consists of two co-axial cylinders in front of the sample. The inner cylinder is held at a positive potential and the outer cylinder at a negative potential. Only the electrons with the right energy can pass through this set-up and are detected at the end. The count rates are high but the resolution (both in energy and angle) is poor. A hemispherical analyser   is often used for applications where higher resolution is needed. It consists of two con-centric hemispheres held a different potentials. The electrons enter and leave through slits. Again, only the electrons with the right kinetic energy, the so-called pass energy can pass the analyser. An electrostatic lens-system can be placed in front of the hemispheres in order to focus the electrons into the analyser and to change the angular acceptance. Such an analyser is shown in Fig. .

  
Figure: A hemispherical electron analyser with a lens system.

In the EELS experiment mentioned above two electron monochromators are needed: one to produce a monochromatic beam and one to analyse the scattered electrons. In a typical apparatus one of these monochromators is movable in order to change the scattering geometry and the momentum transfer (see Fig. ).

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