−
10
−
ME12044A
6.
MEASUREMENT
The XAFS spectrum measurement is carried out in the order of monochromator adjustment
→
detector
adjustment
→
preliminary measurement
→
main measurement, following selection of the monochrometer
crystal and placement of the sample. For the operation method of the measurement software, refer to the
MexWin instruction manual (ME13250A).
6.1
Preparations for Measurement
Because the XAFS spectrum measurement consists of measuring the absorption coefficient, it enables
a spectrum to be obtained regardless of the state of the sample, whether be it in a solid phase, liquid
phase, or gas phase. However, the spectrum may be distorted or a major error may occur in the
analysis if the sample thickness is not uniform or dispersion of elements is not uniform. When a
powder sample is to be measured, a simple solution would be to apply it to a piece of Scotch tape. In
such a case, try to apply the sample as evenly as possible.
Once the sample has been prepared, position the monochrometer crystal suited to the absorption-edge
energy of the target element. If multiple candidates are available, choose one based on the energy
resolution or X-ray intensity.
6.2
Adjustment of the Monochromator
Move the goniometer to a higher-energy region (by approximately 200 eV) than the absorption edge of
the target element. Set the divergence slit to 6 mm and the receiving slit to approximately 0.2 mm, and
select Adjust
→
Monochromator. Scan the
ω
-axis. Initially, scan the entire range (-3 to +3
°
) in steps
of 0.01
°
. If a peak is found, scan the range of
±
0.1
°
with that peak in the center, in steps of 0.001
°
,
and set the
ω
-axis to the peak position.
6.3
Adjustment of the Detector
Select Adjust
→
I Counter, and execute HV scan. If a peak is found, enter that peak value in the HV
field of the PHA scan tab, and activate the PHA scan. Enter the values obtained in the HV scan and
PHA scan in the respective fields of the HV/PHA tab, and click on [OK]. This completes the setup of
the I-detector. In addition, set up the I
0
-counter by following a similar procedure.
6.4
Measurement
Open the measurement-condition dialog box by selecting Measure
→
Execute. The equipment comes
with the measurement conditions set for K-absorption edges from Ti to Ag and some of the
LIII-absorption edges that can be measured using the Looper. Specify the name of the element in the
condition field to invoke the corresponding measurement conditions. Set the measurement time at each
point to one second, and conduct preliminary measurement. If the preliminary measurement results are
satisfactory, extend the measurement time and execute the main measurement. As a rough guide, this
measurement time should be set such that a million counts will be taken where the X-ray intensity is
the lowest. If too much time is required to perform the measurement, then change the settings of the
optical system. For example, change the selection of the monochromator crystal, widen the slit, or the
like. If the preliminary measurement results are abnormal, it is likely that the optical system is not
tuned up properly. Perform the adjustment.
