Nanospectroscopy beamline - LEEM operation mode

LEEM IMAGING

The sample is illuminated with electrons. IL and P1 image the specimen image produced by the objective. The specimen image is finally projected onto the detector by the action of P2 and P3. The contrast aperture in the diffraction plane limits the angular acceptance for optimum lateral resolution.

SPELEEM operation: LEEM imaging mode

OPERATIVE INSTRUCTIONS

After alignment, everything is ready to acquire images in the LEEM mode.

You can get sufficent intensity by either increasing the exposure time in of the CCD grabber, or increasing the emission on the e-gun. To do this reduce the Wehnelt voltage in absolute value. At a field of view 10 microns, start from an emission of 0.05-0.1 microAmps, and if necessary increase this value.

NEVER SATURATE THE DETECTOR!
NEVER GO ABOVE 1.6 microAmp EMISSION ON THE e-GUN!
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.

The contrast aperture placement is done by a linear manipulator, and two knobs for fine tuning. In principle, if all the alignments are correct, one should be able to find the 3 apertures without going to LEED:

Reduce the start voltage towards 0 (to make the 00 beam more intense). Make sure that the intensity is not too high. If so, increase the Wehnelt voltage in absolute values.
The three apertures, 100um, 30um and 20 um, should be found at approximately at 15, 18 and 21 mm on the linear manipulator.
If it is not possible to find the apertures in this way, then go to LEED mode (of course at higher STV), and place the appropriate aperture on the 00 spot.