|
Energy Filter Imaging Unit
The figure shows a schematical cross section of the IEF.
Principle of the Imaging Energy Filter
Two lens elements L1 and L2 form the preretardation lens. The two grids G1
and G2, the two channelplates and the YAG-screen form the Energy Filter Imaging Unit. The two grids G1 and G2 shape the homogeneous electrostatic field that acts as the high pass energy filter. The
double channelplate intensifies the electrons that are followed by the YAG-screen. The YAG screen either converts the electrons into visible light (when operated at high voltage) or
is used as a collector for pulse counting mode.
The mechanical design of the IEF consists of a small cylindrical chamber
containing the electrostatic lens system for preretardation and a special imaging unit containing the retarding grids fixed at the chambers end. It will be
already mounted to the FOCUS PEEM when prepared for shipment. The chamber is screwed to the end of the drift tube of the PEEM and the special
imaging unit is docked on at the other end of that chamber and can be seen behind the view port.
The mounting flange of the IEF chamber is a zero-length adaptor flange
DN63-40CF (conflat flange).This flange is mounted at the FOCUS IS-PEEM’s drift tube (CF35). Any fixing angle (restricted to six positions by the screws) will work. To asure magnetic shielding
, the chamber is covered with a shielding sheet inside the housing. This shielding sheet overlaps with the inner shielding
tube of the PEEM. At the other side of the chamber a DN40CF flange is used to mount the energy filter imaging unit.
The Energy Filter Imaging Unit consists of the two grids followed by a double channelplate and a YAG-screen.
The first grid is mechanically and electrically connected to L2; the second grid G
is kept at the grid potential (provided by the power supply). The two grids form a homogenous electrical field that retards the electrons. All electrons with energies
will be able to pass the second grid and are registered by the imaging unit. The high-pass behaviour is indicated in the Figure.
Electron energies and Voltages in the IEF. The shaded region indicates the part of the
energy-distribution that is transmitted through the second grid.
The electrons in the the shaded region will be able to pass the second grid.
For pulse counting a double channelplate is necessary. The channelplates are
then operated in saturation and all electrons will release high enough pulses to be discriminated by the preamplifier. The YAG-screen is then used as the
collector and kept slightly positive to the channelplate-rear voltage CR.
For imaging the channelplate / YAG assembly is operated similar to the normal
PEEM-mode. Note that due to the double channelplate a higher voltage is necessary to get the same light intensity.
Modes of operation
Imaging Mode
For imaging the screen is connected to the screen high voltage. The transmitted
electrons form the image. An operation similar to the normal PEEM-operation is achieved when the sample is set to (or near) 0V, L1 and L2 to column potential and G to 0V.
Counting Mode
For counting electrons the screen is used as a collector. The charge pulses are decoupled by a high pass and discriminated by the preamplifier.
Spectroscopy
For spectroscopy either the sample or the grid voltage can be varied by the
computer interface. The variation of the sample voltage is preferred for two reasons:
- The electron optics of the objective, that is formed by the sample /
extractor geometry is only slightly changed by changing the sample bias.
- A constant retarding field (L2 - grid voltage) gives a constant energy resolution over the whole spectrum.
|
