Transcript Using a Wavelength Dispersive Spectrometer for EXAFS

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Using a Wavelength Dispersive Spectrometer to measure XAFS
Matt Newville, Steve Sutton, Mark Rivers, Peter Eng
GSECARS
GSECARS beamline and microprobe station, Kirkpatrick-Baez mirrors
XAFS and x-ray fluorescence measurements
The Wavelength Dispersive Spectrometer
Comparison of WDS and solid state detectors
WDS Applications:
XRF:
Cs sorbed onto mica
XANES: Au in FeAsS
EXAFS: Re in K7[ReOP2W17O61].nH2O
GeoSoilEnviroCARS
l
The University of Chicago
l
Argonne National Lab
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The GSECARS Microprobe
The GeoSoilEnviroCARS beamline 13-IDC provides a micro-beam facility
for x-ray fluorescence (XRF) and x-ray absorption spectroscopy (XAS)
studies in earth and environmental sciences.
sample x-y-z stage: 0.1mm step sizes
Horizontal and
Vertical
Kirkpatrick-Baez
focusing mirrors
fluorescence
detector:
multi-element Ge
detector (shown),
Lytle Chamber,
Si(Li) detector,
or Wavelength
Dispersive
Spectrometer
optical microscope (10x to 50x) with video system
GeoSoilEnviroCARS
l
The University of Chicago
l
Argonne National Lab
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Kirkpatrick-Baez focusing mirrors
The table-top Kirkpatrick-Baez mirrors use a fourpoint bender and a flat, trapezoidal mirror to
dynamically form an ellipsis. They can focus a
300x300mm monochromatic beam to 1x1mm - a flux
density gain of 105.
With a typical working distance of 100mm, and an
energy-independent focal distance and spot size,
they are ideal for micro-EXAFS.
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GeoSoilEnviroCARS
l
The University of Chicago
l
Argonne National Lab
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X-ray Absorption Spectroscopy: XANES and EXAFS
Measure the energy-dependence of the x-ray absorption coefficient m(E) [either
log(I0 /I) or (If / I0 )] through a core-level energy of a selected element.
XANES = X-ray Absorption Near-Edge Spectroscopy
EXAFS = Extended X-ray Absorption Fine-Structure
Characteristics of XANES and EXAFS:
Element Specific:
all elements (with Z>20 or so) can be measured at APS
Low Concentration:
selected element can be as low as a few ppm
Natural Samples:
crystallinity is not required -- samples can be
liquids, amorphous solids, soils, aggregates, and surfaces.
Local Structure:
EXAFS gives atomic species, distance, and number
of near-neighbor atoms around selected element
Valence Probe:
XANES gives chemical state and formal valence of selected element
GeoSoilEnviroCARS
l
The University of Chicago
l
Argonne National Lab
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X-ray Absorption Fine-Structure Spectroscopy
1. An x-ray of energy E is absorbed by an atom, destroying a core electron with energy E 0,
and creating a photo-electron with energy (E-E0).
2. The probability of absorption m(E) depends on the overlap of the core-level and photoelectron wave-functions. Since the core-level is localized, this overlap is determined by the
photo-electron wave-function at the center of the absorbing atom. For an isolated atom ,
this is a smooth function of energy.
3. With another atom nearby, the
photo-electron can scatter from the
neighbor. The interference of the
outgoing and scattered waves alters
the photo-electron wave-function at
the absorbing atom, modulating m(E).
4. The oscillations in m(E) depend on
the near-neighbor distance and
species (the energy-dependence of the
scattering amplitude depends on Z).
GeoSoilEnviroCARS
l
The University of Chicago
l
Argonne National Lab
Typical GSECARS Microprobe Application: XRF / EXAFS Sr in coral
Nicola Allison, Adrian Finch (Univ of Brighton, Univ of Hertfordshire, UK)
A common use of the microprobe is to make an x-ray fluorescence (XRF) map
and then collect XANES or EXAFS on selected spots in that map.
The abundance of Sr in aragonite (CaCO3) formed by
corals has been used as an estimate of seawater
temperature and composition at aragonite formation.
Ca
XRF maps of a section of the coral were made with a
5mm X 5mm beam and a 5mm step size. The Sr and Ca
fluorescence (and several other trace elements) were
measured simultaneously at each pixel with a multielement Ge solid-state detector.
Sr
The Sr and Ca maps show incomplete correlation. The
relative Sr abundance therefore varies substantially on
this small length scale, although this section of aragonite
must have been formed at constant temperature.
300mm
The Sr XAFS was measured at a spot with fairly high Sr concentration -- above the
solubility limit of Sr in aragonite...
GeoSoilEnviroCARS
l
The University of Chicago
l
Argonne National Lab
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Typical GSECARS Microprobe Application: XRF / EXAFS Sr in coral
Since the Sr concentration was above its
solubility limit (~1%) in aragonite, it was
not known if Sr would precipitate out into
strontianite (SrCO3: a structural analog of
aragonite), or remain in the aragonite
phase.
First shell EXAFS is same for both
strontianite and aragonite: 9 Sr-O bonds
at ~2.5A, 6 Sr-C at ~3.0A.
Second shell EXAFS clearly shows SrCa (not Sr-Sr) dominating, as shown at
left by contrast to SrCO3 data, and by
comparison to a FEFF-simulated EXAFS
spectrum of Sr substituted into aragonite.
The coral is able to trap Sr in aragonite at
a super-saturated concentration.
GeoSoilEnviroCARS
l
The University of Chicago
l
Argonne National Lab
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Fluorescence XAFS measurements
XRF and XAFS in natural and heterogeneous samples can be complicated by the
presence of fluorescence lines from other elements near the line of interest. A
detector with some energy-resolution helps discriminate against photons at
uninteresting energies.
Fe K-edge
Fe Ka line
Fe Kb line
Example: a dilute quantity of Co in an Fe-rich system.
The Fe will be excited by the Co K-edge radiation.
Even though the Fe Kb is 5X weaker than the Fe Ka Co K-edge
intensity, it may be much larger than the Co Ka intensity. Co Ka line
7.112 KeV
6.403 KeV
7.057 KeV
7.709 KeV
6.930 KeV
Similar conflicts occur when two L lines interfere with each other (the La and Lb are
about the same intensity, too), or when an L and a K-line interfere.
Si(Li) and Ge solid-state detectors give energy resolutions of ~100 to ~300 eV
(with the best resolution often limiting count rates to ~1KHz), which is sometimes
not good enough.
These detectors are also limited in total count rate (up to ~100KHz, but at the
worst resolution), which can be a problem -- especially with intense x-ray beams.
GeoSoilEnviroCARS
l
The University of Chicago
l
Argonne National Lab
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The Wavelength Dispersive Spectrometer (Oxford WDX-600)
Borrowing technology developed for the electron-microscope community, the
Wavelength Dispersive Spectrometer uses an analyzer crystal on a Rowland
circle to select a fluorescence line. This has much better resolution (~30eV)
than a solid state detector (~250eV), doesn’t suffer from electronic effects like
dead-time, and can have superior peak-to-background ratios. The solid-angle
and count-rates are somewhat lower.
Sample and x-y-z stage
Kirkpatrick-Baez focusing mirrors
Ion chamber
Table-top slits
Optical microscope
Wavelength Dispersive Spectrometer
GeoSoilEnviroCARS
l
The University of Chicago
l
Argonne National Lab
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210mm Rowland circle containing
sample, crystal analyzer, and detectors
GeoSoilEnviroCARS
l
The University of Chicago
l
Argonne National Lab
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WDX-600: detailed view
detectors = 2 proportional counters:
(one flowing P-10 gas, and one sealed
with 2 atm Xe) in tandem.
slits: define angular acceptance and
energy resolution
crystals = LiF (200), LiF(220), LiF(420),
and PET, on a six crystal turret. Crystal
size ~45 x 15 mm
By using a Johannson geometry
Rowland circle, a point source
focuses to a point at the detector slit.
Aberrations are minimized, and the
signal-to-noise ratio is improved.
GeoSoilEnviroCARS
l
The University of Chicago
l
Argonne National Lab
Comparisons of the WDS and solid-state detectors
Steve Sutton and Mark Rivers, data collected at NSLS X-26A.
Here’s part of the XRF spectra for a synthetic glass containing
several rare-earth elements using both a Si(Li) detector and the WDS.
GeoSoilEnviroCARS
l
The University of Chicago
l
Argonne National Lab
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Comparisons of the WDS and solid-state detectors
Typical values for the WDS and a Ge solid-state detector
Ge Solid-State
WDS
energy resolution:
~30eV
~100eV to ~300eV
depending on shaping time
active area:
~500mm2`
(varies with angle)
100mm2 (per detector, often 13X)
working distance:
~180mm
~100mm
max total count rate:
GeoSoilEnviroCARS
none
l
100KHz (per detector, often 13X)
The University of Chicago
l
Argonne National Lab
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Issues using the WDS
Alignment:
The WDS weighs ~30kg, and needs to be aligned fairly well:
~1 mm vertical
~1 mm in/out-board
~10mm up/down-stream
For our initial run, we adjusted the height by hand, and had a motorized
in/out-board motion. For the up/down-stream position, we brought the
sample to the spectrometer, which limits the focusing ability of the microprobe.
Tunability: The WDS selects one energy at a time, and looking at different
energies requires a mechanical scan. So, unlike a solid-state
detector, the WDS does not simultaneously measure multiple
energies --- it does not have an MCA.
So XRF maps of multiple elements (like the Sr/Ca example) are
not practical with the WDS.
GeoSoilEnviroCARS
l
The University of Chicago
l
Argonne National Lab
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Using the WDS for XRF: Cs on biotite
S Sutton, J McKinley, J Zachara (PNNL)
Biotitie is a mica that contains trace amounts
of many transition metals, a few percent Ti,
and major components of Ca and Fe.
To study how Cs would bind to the surface
and layers of the biotite, McKinley and
Zachara exposed a cross-cleavage plane of
biotite to a Sr-rich solution.
With a solid-state detector, the Cs La line (at
4.286KeV) was a small shoulder on the Ti Ka
line (at 4.510KeV), making a map of Cs
concentration from the La intensity was
impossible. Mapping with the Cs K-edge was
not useful either (x-rays too penetrating into
bulk mica, and too much inelastic scattering).
The map at right shows the Cs concentration
as measured with the WDS on the Cs La line.
GeoSoilEnviroCARS
l
100 x 100mm image, with a 5 x 5mm
beam, taking 3mm steps, with a 30s
dwelltime at each point. The incident
x-ray energy was 10KeV.
The University of Chicago
l
Argonne National Lab
16
Using the WDS for XANES: 1000ppm Au in FeAsS (arsenopyrite)
Louis Cabri (NRC Canada), Robert Gordon, Daryl Crozier (Simon Fraser), PNC-CAT
1000ppm Au in FeAsS (arsenopyrite): The understanding of the chemical and
physical state of Au in arsenopyrite ore deposits is complicated by the proximity of
the Au LIII and As K edges and their fluorescence lines.
At the Au LIII-edge, As will also be excited, and fluoresce near the Au La line.
As K-edge
As Ka line
11.868 KeV
10.543 KeV
Au LIII-edge
Au La line
11.918 KeV
9.711 KeV
Even using the WDS, the tail of
the As Ka line persists down to
the Au La line, and is still
comparable to it in intensity.
GeoSoilEnviroCARS
l
The University of Chicago
l
Argonne National Lab
17
Using the WDS for XANES: 1000ppm Au in FeAsS (arsenopyrite)
Louis Cabri (NRC Canada), Robert Gordon, Daryl Crozier (Simon Fraser), PNC-CAT
With a 13-element Ge detector (at PNC-CAT:
ID-20), the tail of the As Ka line was still strong
at the Au La energy, so the Au LIII edge-step
was about the same size as the As K edgestep, and the Au XANES was mixed with the
As EXAFS.
With the WDS, the As edge was visible, but
much smaller, so the Au XANES was clearer.
Measuring two different natural samples of
FeAsS, both with ~1000ppm of Au, we see
evidence for both metallic and oxidized Au.
GeoSoilEnviroCARS
l
The University of Chicago
l
Argonne National Lab
18
Using the WDS for EXAFS: Re in K7[ReOP2W17O61].nH2O
Mark Antonio (ANL)
The inorganic molecule a-P2W17O61 is a candidate for stabilizing transition and
rare-earth metal ions. It can lose a WO ligand and replace it with several
valence states of Re (a nice, safe chemical analog of Tc).
The proximity of the Re and W LIII-edges, and their La
lines, and the relative concentrations of Re and W
(1::61) in this sample makes EXAFS measurements
using a solid-state detector nearly impossible.
W LIII-edge
W La line
10.204 KeV
8.396 KeV
Re LIII-edge
Re La line
10.534 KeV
8.651 KeV
Venturelli, et al, J. Chem. Soc., Dalton Trans., p 301 (1999)
GeoSoilEnviroCARS
l
The University of Chicago
l
Argonne National Lab
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Using the WDS for EXAFS: Re in K7[ReOP2W17O61].nH2O
Here are m(E), the EXAFS kc(k), and the
Fourier transform of the EXAFS |c(R)| for
data collected with the WDS. The data is
the average of 3 scans, each having an
integration time of 5 seconds per point.
The data quality is acceptable up to ~12A-1,
and initial analysis supports a first shell with
4 oxygens at 1.8A.
GeoSoilEnviroCARS
l
The University of Chicago
l
Argonne National Lab
20
Using a Wavelength Dispersive Spectrometer to measure XAFS
The Wavelength Dispersive Spectrometer can be used for XANES and
EXAFS measurements. In some cases it is sometimes the only detector
capable of such measurements.
In many cases, the WDS compares favorably with solid state detectors.
In some cases, the WDS is superior to solid-state detectors, and is the only
detector capable of XRF, XANES, and EXAFS measurements.
GeoSoilEnviroCARS
l
The University of Chicago
l
Argonne National Lab