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Photoemission with synchrotron radiation (NEXAFS, RPES)

These measurements include those where the photon energy is equidistantly scanned, e,g. secondary-electron-yield (SEY) and Auger-electron-yield (AEY) modes of X-ray absorption (XAS, NEXAFS, XANES) or resonant photoemission (RPES). For this we use the KolXPD software that can control both electron analyzer and beamline monochromator. Scans that are non-equidistant in photon energy can be performed manually (i.e. region after region) in SpecsLab2 as described here.

The most typical possibilities are listed in the following table:
method mode scanning measuring analyzer axis scope analyzer
mode
Epass stray features
RPES   photon energy core level or valence band spectrum binding energy spectrum shape Standard 2–20 eV Auger lines, 2nd order core levels
XAS,
NEXAFS,
XANES
SEY photon energy secondary electrons kinetic energy total intensity Smart 1–2 eV 1st and 2nd order core levels
AEY photon energy Auger line kinetic energy total intensity Smart 20–50 eV 1st and 2nd order core levels


Measurements of RPES

Data acquisition in KolXPD is similar to that of SpecsLab2 described here, with following differences:
  • You cannot change the number of scans when the acquisition is already running.
  • You cannot add more regions into the acquisition queue when the acquisition is already running.
  • Instead of .xml files .exp file format is used. All analyzer channels and scans of every region are summed and cannot be separated.
  • Remaining time is not shown.
The procedure is as following:
  1. Switch on analyzer power supply and detector electronics (in the lower part of the right experimental rack) if not yet done.
  2. Open KolXPD.
  3. Check that the analyzer is disconnected from SpecsLab2 and connect KolXPD to the analyzer electronics using the Connect button. Wait until the analyzer is connected. If an error message appears proceed with Electron analyzer connection troubleshooting.
  4. Open an older file to repeat the measurements with the same or similar parameters, or create a new one by selecting New button - File and New - CIS / CFS / NEXAFS / RESPES. Data in old files are cleared by a Clear button.
  5. Select the photon energy range From-To and Step in the Monochromator - Scan: Energy section. Be aware that the photon energy range might be slightly different due to miscalibration so it might be convenient to verify it by measuring reference samples. Validate the setting by clicking on Accept.
  6. Set the region parameters similar to those used in SpecsLab2 as described here. Namely:
    • Energy axis: Binding, excitation energy: any number of you photon energy range
    • Start, End, Epass, Dwell, Step and Sweeps equally to SpecsLab2's Energy Start, Energy End, Epass, Dwell time*1000, Energy Step and Scans, respectively.
    • Lens Mode: MediumArea:400V or MediumArea:1.5kV equally to what you would set in SpecsLab2.
    • Detector voltage 1825 V and Work function 4.36 eV.
  7. Set correctly the sample position (polar angle Θ included) according to the previous alignment in SpecsLab2 and check it throught the analyzer viewport.
  8. Open the electropneumatic valve 10 of the beamline.
  9. If desired you can check the parameters by acquiring only the selected region by clicking on the Start button. Modify the parameters if necessary.
  10. Run the whole photon energy scan by clicking on the Start button. By observing the progress you can estimate the remaining time.
  11. At the end of the acquisition do not forget to Save your data and close the electropneumatic valve 10 of the beamline.
  12. Disconnect the analyzer (using the Disconnect button) if you plan to switch off the electronics or to use SpecsLab2 for following acquisition, as only one software, i.e. either KolXPD or SpecsLab2, can be connected to the electronics at the same time.


Measurements of XAS, NEXAFS, XANES

It is in principle similar to that of RPES described above, with these two exceptions:
  • The regions (secondary electrons in SEY or Auger lines in AEY) are acquired with constant kinetic energy range, not binding.
  • In order to speed-up the measurements (thanks to multichannel detection) at the cost of the fidelity of the spectrum shape, Smart mode (also called snapshot mode) is used instead of Standard. In Standard mode the analyzer is scanning the energy range by 20% of Epass broader than required by the End-Start setting, in order to have the intensities in every energy point collected by all 100 channels of the detector; some collected intensities are therefore thrown away. Contrary to it, in the Smart mode, the software is using the energetically-dispersed channel data in order to cover the desired End-Start energy range (20% of Epass). For example, with Epass = 20 eV the region 4 eV broad can be measured with a single Snapshot and the average energy step will be 0.04 eV (i.e. 4 eV / 100 channels). If 4 eV range is not sufficient more Snapshot can be measured, preferably partially overlapping in order to partially smooth the discontinuities on the snapshot edges (but it is not important, as we are interested only in the total intensity, i.e. sum of all channels) . However, In order to have at least some information about the spectral shape, at the beginning of each photon energy scan KolXPD measures the selected number of scans (Sweeps) in the Standard mode (at Excitation  energy selected) to calibrate the channel sensitivities, and then proceeds in the faster Smart mode.


Normalization to photon flux

When measuring photon energy scans in KolXPD, mesh current is automatically acquired and stored in the data files. In can be used for normalization of the spectra before exporting them. However, mesh current is reflecting the real photon flux only in the ranges where the used  (C-contaminated) Au mesh is not absorbing light, in our case far fron Au and C absorption edges. Therefore, for processing of C K-edge spectra another normalization procedure must be used. For example blank spectrum acquired on any carbon-free sample or the flux curve obtained from a photodiode, preferably with the same monochromator and beam conditions/settings.

Overlapping Auger or core levels

When setting up the regions to be acquired, be careful that stray features can overlap your spectra during photon energy scans. Typically they are core levels excited with 2nd order light (double photon energy), Auger lines overlaping core levels or core levels overlapping Auger lines. All these features appear on the 2D E vs. Excitation image in KolXPD as inclined lines that are crossing the horizontal and vertical lines reflecting the desired spectral features.

Some of these stray features can be removed during post-processing e.g. by using Igor macro EccentricXPS. For details see Removing photoemission features from Auger-yield NEXAFS spectra (J. Electron Spectroscopy and Related Phenomena 218, 2017, 35-39).


Last Updated on Tuesday, 27 February 2018 15:14