HowTo&Troubleshooting

How to perform XAS measurements

During the alignment of the beamline, in the energy selection and, of course, in the measurements it is always better to have the best possible statistics. One important setting concerns the gain of the electrometers or amplifiers (the Keithleys) used to collect both I0 (address 25) and IS (address 24). In fact, these instruments convert small currents (down to fA) to voltages that are then read by the acquisition board close to the main beamline computer.

TEY

IN BRIEF:

• Remote PreAmp IS connected (if the sample is not grounded!)
• TEY COUNTER SETTINGS: I0=ON;IS=ON;IREL=ON

  PRACTICAL PROCEDURE:
  0. MainPanel.VI to set the required Energy Resolution by selecting the appropriate opening of the Entarnce and Exit slit.
  Press =>. Select the Element (Entrance Slit), the Action (Opening). Set the value (absolute) between 10 and 50 and press MOVE. Select the Element (Exit Slit), the Action (Opening). Set the value (absolute) between 10 and 50 and press MOVE.

  1. Remote PreAmp. Check that there is overload between the BNC pin and the ground. If there is overload and no finite resistance, connect the Remote PreAmp to the BNC.
  CAUTION!!! Before connecting the Keithley Remote PreAmp (see picture: more details) to the BNC on top of themanipulator to collect Is signal, check that there is overload between the BNC pin and the ground. If there is a finite resistance you can seriously damage the keithley!!! For the same reason ALWAYS disconnect the Remote PreAmp from the BNC, after having pressed OFF (on the main panel under the blue ligth, the blue ligth will get off) when you need to transfer the sample because the transfer arm grounds the sample. In anycase contact BL staff for more information.

  2. Set Energy and Undulator.VI. Select an energy about 10 eV higher then the position of the edges you want to measure. Use First harmonic when possible (for linear horizontal polarization at 2.0 GeV up to about 730 eV, at 2.4 GeV up to about 800 ?eV) or second harmonic. In order to see if you can use the fist harmonic, use Set Energy and Undulator and try to Calculate the Ideal gap before pressing Set ID and E. If you find Ideal gap as a value between 19.1 and 49.9 then the 1st harmonic can be used. Otherwise choose the 2nd harmonic. The third harmonic has more intensity, but it is usually too steep to perform XAS.

  3. Main Panel. VI (or Keithley Initialise 6430.VI) Initialise both 24 and 25 to the best scale .Select the the lowest rangepossible without causing an overflow to ensure best accuracy and resolution. The signal saturates at 1.05.
  For example, initialise both I0 (and IS) first to 1x10^-6. Then read the counters using Counter read all.VI. Finally set the optimal scale (for instance if you read Io=4x10^-9 on top of the harmonic, you should initialise the address=25 to 1x10^-8.possible without causing an overflow to ensure best accuracy and resolution (more details).

  4. Scan Energy GALIL.VI. a) Counter Setting: Check that both I0 (and eventually IS and IREL for TEY if the sample is insulated) are selected (ON).
  Note: all the scanning VI (Scan energy, scan motor, scan XPS, scan resonant XPS,..) have to be STOPPED before using Counter setting.
  (b) The ENERGY STEP depends on the energy and grating. One good starting value can be 0.15 eV. However, if the monochromator is not able to perform regular steps, this means that the chosen step is too small. Typical CYCLES per point is from 5 to 35. DO NOT USE NUMBER OF CYCLES HIGHER THAN 35.
  The scan will be saved point by point: even if the VI is stopped before the end of the scan, the scan will be saved.

   

  PEY

  IN BRIEF:

• Remote PreAmp IS OFF. 2) Remote PreAmp IS disconnected. 3) Ground cable connected.
• PEY: COUNTER SETTINGS: I0=ON;XPS=ON (time=1000 or more);IS=OFF;IREL=OFF.
  Note: all the scanning VI (Scan energy, scan motor, scan XPS, scan resonant XPS,..)have to be STOPPED before using Counter setting.VI

  PRACTICAL PROCEDURE:
  0. MainPanel. VI to set the required Energy Resolution.
  Press =>. Select the Element (Entrance Slit), the Action (Opening). Set the value (absolute) between 10 and 50 and press MOVE. Select the Element (Exit Slit), the Action (Opening). Set the value (absolute) between 10 and 50 and press MOVE.
  
  1. Set Energy and Undulator.VI . Select an energy about 10 eV higher then the position of the edges you want to measure. Use First harmonic when possible (at 2.0 GeV up to about 730 eV, at 2.4 GeV up to about 800 ?eV) or second harmonic.

  2. Keithley Initialise 6430.VI. Initialise both 24 and 25 to the best scale .Select the the lowest rangepossible without causing an overflow to ensure best accuracy and resolution. The signal saturates at 1.05.
  For example, initialise both I0 first to 1x10^-6. Then read the counters using Counter read all.VI. Finally set the optimal scale (for instance if you read Io=4x10^-9 on top of the harmonic, you should initialise the address=25 to 1x10^-8.possible without causing an overflow to ensure best accuracy and resolution (more details).

  3. XPS onoff.VI: Turn ON XPS. (Select ON, Press inside the circle until a point appears, Press=>). Then Press OK in the Popup window and wait until the green buttom gets red).

  4. XPS Survey.VI. Select the secondary electrons: Max KE=30;PE=90;Time=1000; Run the VI and when the KE has been set, press STOP.

  5. Scan Energy GALIL.VI. (a) 1. Counter Setting.VI. Check that both I0 and XPS are selected (ON).
  Note: all the scanning VI (Scan energy, scan motor, scan XPS, scan resonant XPS,..) have to be STOPPED before using Counter setting.VI.
  (b) Energy step depends on the energy and grating. One good starting value can be 0.15 eV. However, if the monochromator is not able to perform regular steps, this means that the chosen step is too small. Typical Cycles per point is 5. The scan will be saved point by point: even if the VI is stopped before the end of the scan, the scan will be saved..