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Using LEED



Using LEED

  1. Check that electron analyzer, X-ray source, evaporators, titanium sublimation pump etc. are off and that all gate valves to other chambers are properly closed.
  2. Set the sample position to approximately x = 0 to 5, y = 24, z = 239, Θ = 90°. As x is far from the centre it should be set as the last axis.
  3. Check that the knobs of screen, energy and filament are set to zero (completely counterclockwise).
  4. Switch on the control unit.
  5. Slowly increase filament current up to 1.25 A (IFIL). The emission should reach at least 0.3 mA (IE).
  6. Increase the high voltage on the screen to 2, 3 or 4 kV.
  7. Rotate the energy knob in order to observe spots on the screen.
  8. If the spots are too intense even with the screen voltage 2 kV decrease the filament current to 1.1 or 1.0 A.
  9. You can use the camera for observing the patterns on the computer display and save them into files using the KolLEED software. The procedure is described on the next page.
  10. Check the sample position and the surface homogeneity by scanning the sample surface in the y (horizontal) and z (vertical) directions. Polar angle Θ tilts the sample horizontally; when the beam is perpendicular the [00] spot is behind  the electron gun housing. Vertical tilt can be adjusted only by tilting the sample on the sample holder during mounting.
  11. After the acquisition set the energy to zero, switch off the screen voltage, slowly set filament to zero.
  12. Switch off the control unit.
  13. Move the sample to a safe position. Start with setting x to approximately 25 (centre of the chamber).















Adjustment

If no spots are visible but you are sure that the sample is crystalline and the position is correct, try adjusting suppressor, wehnelt or lens (L1,3, L2) voltages (offsets, gains). The usual setting (4 Nov 2017) is:
  • supressor at about one third (black mark on the control unit)
  • wehnelt at zero
  • energy L2 L1,3  
    0 30 18 adjust with offset
    200 81 463 adjust with gain and repeat
If the spots suddenly disappear there could be a problem with the filament. Sometimes it gets misaligned when on for long time. Try to switch everything off for several minutes to cool it down and try again.

Last Updated on Monday, 11 May 2020 13:35