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Sample alignment with respect to the analyzer and photon beam

Alignment with X-ray source (by eye)

As the area illuminated by the X-ray source is huge, the alignment with respect to the analyzer and illumination is not critical.

General rules are:
  • sample is visible in the center while looking through the analyzer viewport;
  • sample-to-analyzer distance is 40 mm.
Be aware that the diameter of the analyzed spot can be up to several milimeters (see graph) so other materials can be detected around small samples.

If you want to be sure (and the synchrotron beam is available), you can align the sample using the synchrotron beam (as described below but at Θ = 190°) and then switch to X-ray source excitation, while keeping the same sample position.

Alignment with synchrotron beam (with SpecsLab2 timescan)

As, during synchrotron-excited measurements, the photon beam size is below 0.25 mm, it is critical to find the correct sample position with respect to the beam and the analyzer. It supposes that the synchrotron beam position is correct and stable (beamline opened and mirrors warmed up for at least 30 min, preferably several hours). The sample must be homogeneous, conductive and grounded; otherwise see Problematic samples below.
  1. Switch on analyzer power supply and detector electronics (in the lower part of the right experimental rack) if not yet done.
  2. Open SpecsLab2.
  3. Check that the analyzer is disconnected from KolXPD and connect SpecsLab2 to the analyzer electronics by selecting Analyzer: Phoibos-Hsa3500 in the button bar. Wait until the analyzer is connected. If an error message appears proceed with Electron analyzer connection troubleshooting.
  4. Open an older file with alignment or create a new one. Software controls are similar as during usual synchrotron data acquisition, except for the Region parameters::

    • Method: UPS
    • Analysis Mode: FixedEnergies
    • Lens Mode: MediumArea
    • Range:400V
    • Eexc: 50–200 eV recommended
    • Energy: kinetic energy where the timescan will run
      (always kinetic, even when binding energy scale is
      displayed in the graph)
    • Epass: 2–10 eV; keep intensity below 1 Mcps
    • Values: 2000
    • Scans: 1
    • Dwell Time: 0.1 or 0.2 s
    The example above shows alignment on the intensity in the valence band spectrum (i.e. Ebin 5 eV = Eexc 150 eV - Energy 145 eV) which usually works well for most of the samples. Try another Energy if your sample has no occupied states in this binding energy region.
  5. Set the sample position (x,y around 25, z around 113, Θ usually 210° - normal emission) in order to see the sample in the center of the analyzer viewport. use lamps or torch to see.
  6. Open the beamline electropneumatic valve 10 and start the timescan acquisition (Validate, Acquire).
  7. By adjusting x,y and observing through the analyzer viewport try to reach the maximum intensity on the timescan while keeping the sample visibly centered. Perfect alignment means that the intensity drops from the maximum symetrically to the same value if you misalign y by +0.5 or by -0.5 mm.
  8. Adjusting z does not influence the alignment but it can be used to scan the sample surface vertically. For example, if you want to be vertically in the center, move z up and down to find sample borders zupper, zlower (discontinuities will appear on the intensity timescan) and then set z to the average of both values.
  9. (Sometimesit can be useful to check also the refocusing mirror pitch: the intensity on the timescan should drop from the maximum symetrically to the same value if you misalign the pitch by +50 and -50 μm on the corresponding micrometer.)
  10. Write down the determined correct position on the sample.
  11. Repeat points 7 and 10 for other emission angles you want to use in your experiments, e.g. Θ = 230° (grazing incidence), 190° (20° off-normal emission in XPS), 185.3° (magic angle), 150° (normal incidence SR, 60° off-normal emission in SR or XPS).
  12. At the end of alignment procedure switch off the lamps illuminating the chamber inside, as such light might influence the shape of your acquired spectra on some photosensitive samples (typically Si wafers).
It is recommended to check the alignment everytime the position of the photon beam or of the sample might have changed (new injection after a beam dump, orbit variations, beamline warmup, sample holder removal from the manipulator...).

Problematic samples

If you cannot get any intensity on the timescan even when you move the sample, check that the electropneumatic valve 10 is open, beam is available (mesh current > 0) and the alignment is not completely wrong by eye. In case of some types of sample the alignment can be very difficult or even impossible:
  • If you keep observing very weak or zero intensity on the timescan, it is usually caused by strong charging. Measurements of such samples with synchrotron radiation is impossible (but you might try it with X-ray source}. Always use sufficiently electrically conductive samples and ground them properly.
  • If the intensity on the timescan is unstable in time and shows a peak immediately after you change the sample position, it is usually caused by weak charging. Measurements of such samples with synchrotron radiation might be impossible.
  • If you cannot find clear intensity maximum on the timescan, it might be that the sample composition is inhomogeneous, which can be already visible by eye during sample mounting. In this case you have to play more with the sample position and probably acquire your spectra in more different spots on the sample to characterize it.
  • If the sample is too tiny for reasonable alignment, you may try the so-called 30/30 configuration (30° off-normal incidence and 30° emission angle; Θ = 180° on the manipulator). In such case y is affecting only the focus of the analyzer with respect to the photon beam, and x is affecting only the horizontal position on the sample (similarly to z for vertical position in point 8 above). So by moving x you can move the sample horizontally in order to find discontinuities on the intensity timescan (xleft and xright) and set x to their average, indicating the sample center. Of course, at Θ = 180° the total intensity is lower than it would be Θ = 210° (normal emission) but at least you can be sure about the spot position.
Last Updated on Friday, 23 February 2018 14:23