Nanospectroscopy beamline - Diffraction operation mode

The sample is illuminated with low energy electrons, or soft x-rays or UV radiation. IL and P1 image the diffraction pattern produced in the backfocal plane of the objective lens. Inserting the analyzer exit slit allows angle resolved photoemission and x-ray photo-electron diffraction measurements. The probed area is selected by inserting the field-limiting aperture in the image plane after the objective lens, the contrast aperture is removed.

$micro-diffraction$
SPELEEM operation: micro-diffraction mode

OPERATIVE INSTRUCTIONS

In diffraction mode, especially when using LEED, one has to be very careful not to burn the detector with the intense diffraction peaks at very low energy.

NEVER SATURATE THE DETECTOR!
BE CAREFUL NOT TO PUT TOO MUCH INTENSITY!
Do not use more than 10% of the full camera range at exposure of 0.02 ms. In emergency, turn off the 18 kV high voltage, and check all the settings.

Before operating the microscope in diffraction mode:

Start the grabber preview in LEEM/XPEEM mode.
Insert the smallest field-limiting aperture (around 21 mm on the feedthrough) in the centre of the area imaged by LEEM/XPEEM.
Set the Wehnelt so that emission is zero (0.01 microAmps on the meter).
Remove the contrast aperture.
Set the start voltage (STV) to a high value (such as 30 V).
Insert the energy slit for ARUPS, XPD or if you want to filter secondary electrons in LEED.

Then simply click on LEED button in UMeasure (same in LEEM2000).

LEED

Carefully ncrease the emission current. Check if the intensity on the detector is too high. Don't go above 0.3 µA emission!!

If the illuminated circle (cut from the Ewald sphere) is not round, adjust the DIFFRACTION STIGMATORS.

If the pattern is not centered to the Ewald sphere adjust the ILLUMINATION DEFLECTORS.

To go back to LEEM, simply click on the corresponding field-of-view button and remove the field-limiting aperture.