Presentation on theme: "GeoSoilEnviroCARS The University of Chicago Argonne National Lab 1 Using a Wavelength Dispersive Spectrometer to measure XAFS Matt Newville, Steve."— Presentation transcript:
GeoSoilEnviroCARS The University of Chicago Argonne National Lab 1 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 EXAFS:Re in K 7 [ReOP 2 W 17 O 61 ].nH 2 O Comparison of WDS and solid state detectors XANES:Au in FeAsS WDS Applications: XRF: Cs sorbed onto mica
GeoSoilEnviroCARS The University of Chicago Argonne National Lab 2 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. Horizontal and Vertical Kirkpatrick-Baez focusing mirrors fluorescence detector: multi-element Ge detector (shown), Lytle Chamber, Si(Li) detector, or Wavelength Dispersive Spectrometer sample x-y-z stage: 0.1 m step sizes optical microscope (10x to 50x) with video system
GeoSoilEnviroCARS The University of Chicago Argonne National Lab 3 Kirkpatrick-Baez focusing mirrors The table-top Kirkpatrick-Baez mirrors use a four- point bender and a flat, trapezoidal mirror to dynamically form an ellipsis. They can focus a 300x300 m monochromatic beam to 1x1 m - a flux density gain of With a typical working distance of 100mm, and an energy-independent focal distance and spot size, they are ideal for micro-EXAFS. We routinely use Rh-coated silicon (horizontal) and fused-silica (vertical) mirrors to produce 4x4 m beams for XRF, XANES, and EXAFS.
GeoSoilEnviroCARS The University of Chicago Argonne National Lab 4 X-ray Absorption Spectroscopy: XANES and EXAFS Element Specific:all elements (with Z>20 or so) can be measured at APS Measure the energy-dependence of the x-ray absorption coefficient (E) [either log(I 0 /I) or (I f / I 0 )] through a core-level energy of a selected element. Natural Samples:crystallinity is not required -- samples can be liquids, amorphous solids, soils, aggregates, and surfaces. Characteristics of XANES and EXAFS: Low Concentration:selected element can be as low as a few ppm 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 XANES = X-ray Absorption Near-Edge Spectroscopy EXAFS = Extended X-ray Absorption Fine-Structure
GeoSoilEnviroCARS The University of Chicago Argonne National Lab 5 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-E 0 ). 2. The probability of absorption (E) depends on the overlap of the core-level and photo- electron 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 (E). 4. The oscillations in (E) depend on the near-neighbor distance and species (the energy-dependence of the scattering amplitude depends on Z).
GeoSoilEnviroCARS The University of Chicago Argonne National Lab 6 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 (CaCO 3 ) formed by corals has been used as an estimate of seawater temperature and composition at aragonite formation. XRF maps of a section of the coral were made with a 5 m X 5 m beam and a 5 m step size. The Sr and Ca fluorescence (and several other trace elements) were measured simultaneously at each pixel with a multi- element Ge solid-state detector. 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. 300 m Ca Sr The Sr XAFS was measured at a spot with fairly high Sr concentration -- above the solubility limit of Sr in aragonite m
GeoSoilEnviroCARS The University of Chicago Argonne National Lab 7 Since the Sr concentration was above its solubility limit (~1%) in aragonite, it was not known if Sr would precipitate out into strontianite (SrCO 3 : 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 Sr- Ca (not Sr-Sr) dominating, as shown at left by contrast to SrCO 3 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. Typical GSECARS Microprobe Application: XRF / EXAFS Sr in coral
GeoSoilEnviroCARS The University of Chicago Argonne National Lab 8 Fluorescence XAFS measurements 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 K is 5X weaker than the Fe K intensity, it may be much larger than the Co K intensity. Similar conflicts occur when two L lines interfere with each other (the L and L are about the same intensity, too), or when an L and a K-line interfere. Fe K-edge KeV Fe K line KeV Fe K line KeV Co K-edge KeV Co K line KeV 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. 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 The University of Chicago Argonne National Lab 9 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. The Wavelength Dispersive Spectrometer (Oxford WDX-600) Kirkpatrick-Baez focusing mirrors Ion chamber Table-top slits Wavelength Dispersive Spectrometer Sample and x-y-z stage Optical microscope
GeoSoilEnviroCARS The University of Chicago Argonne National Lab mm Rowland circle containing sample, crystal analyzer, and detectors
GeoSoilEnviroCARS The University of Chicago Argonne National Lab 11 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. WDX-600: detailed view detectors = 2 proportional counters: (one flowing P-10 gas, and one sealed with 2 atm Xe) in tandem. crystals = LiF (200), LiF(220), LiF(420), and PET, on a six crystal turret. Crystal size ~45 x 15 mm slits: define angular acceptance and energy resolution
GeoSoilEnviroCARS The University of Chicago Argonne National Lab 12 Comparisons of the WDS and solid-state detectors 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. Steve Sutton and Mark Rivers, data collected at NSLS X-26A.
GeoSoilEnviroCARS The University of Chicago Argonne National Lab 13 Comparisons of the WDS and solid-state detectors energy resolution: ~30eV ~100eV to ~300eV depending on shaping time active area: ~500mm 2` 100mm 2 (per detector, often 13X) (varies with angle) working distance: ~180mm ~100mm max total count rate: none 100KHz (per detector, often 13X) WDS Ge Solid-State Typical values for the WDS and a Ge solid-state detector
GeoSoilEnviroCARS The University of Chicago Argonne National Lab 14 Alignment: The WDS weighs ~30kg, and needs to be aligned fairly well: ~1 mm vertical ~1 mm in/out-board ~10 m 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. Issues using the WDS 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 The University of Chicago Argonne National Lab 15 Using the WDS for XRF: Cs on biotite S Sutton, J McKinley, J Zachara (PNNL) 100 x 100 m image, with a 5 x 5 m beam, taking 3 m steps, with a 30s dwelltime at each point. The incident x-ray energy was 10KeV. 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 L line (at 4.286KeV) was a small shoulder on the Ti K line (at 4.510KeV), making a map of Cs concentration from the L 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 L line.
GeoSoilEnviroCARS The University of Chicago Argonne National Lab ppm 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 L III and As K edges and their fluorescence lines. At the Au L III -edge, As will also be excited, and fluoresce near the Au L line. Even using the WDS, the tail of the As K line persists down to the Au L line, and is still comparable to it in intensity. Using the WDS for XANES: 1000ppm Au in FeAsS (arsenopyrite) Louis Cabri (NRC Canada), Robert Gordon, Daryl Crozier (Simon Fraser), PNC-CAT As K-edge KeV As K line KeV Au L III -edge KeV Au L line KeV
GeoSoilEnviroCARS The University of Chicago Argonne National Lab 17 With a 13-element Ge detector (at PNC-CAT: ID-20), the tail of the As K line was still strong at the Au L energy, so the Au L III edge-step was about the same size as the As K edge- step, 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. Using the WDS for XANES: 1000ppm Au in FeAsS (arsenopyrite) Louis Cabri (NRC Canada), Robert Gordon, Daryl Crozier (Simon Fraser), PNC-CAT Measuring two different natural samples of FeAsS, both with ~1000ppm of Au, we see evidence for both metallic and oxidized Au.
GeoSoilEnviroCARS The University of Chicago Argonne National Lab 18 Using the WDS for EXAFS: Re in K 7 [ReOP 2 W 17 O 61 ].nH 2 O Mark Antonio (ANL) Venturelli, et al, J. Chem. Soc., Dalton Trans., p 301 (1999) W L III -edge KeV W L line KeV Re L III -edge KeV Re L line KeV The proximity of the Re and W L III -edges, and their L lines, and the relative concentrations of Re and W (1::61) in this sample makes EXAFS measurements using a solid-state detector nearly impossible. The inorganic molecule -P 2 W 17 O 61 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).
GeoSoilEnviroCARS The University of Chicago Argonne National Lab 19 Using the WDS for EXAFS: Re in K 7 [ReOP 2 W 17 O 61 ].nH 2 O Here are (E), the EXAFS k (k), and the Fourier transform of the EXAFS | (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 The University of Chicago 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.