The Evolution of X-ray Absorption Spectroscopy of the ...

The Evolution of X-ray Absorption Spectroscopy of the Actinides

12th School on the Physics and Chemistry of the Actinides 19-21 March 2018 Campus Tecnológico e Nuclear, Instituto Superior Técnico (IST/C2TN), Bobadela, Portugal 19 March 2018

J. G. Tobin

University of Wisconsin, Oshkosh, Oshkosh, WI

e-mail: [email protected]

mailto:[email protected]

12th SPCA, IST/C2TN, Bobadela, Portugal JG Tobin

The Evolution of X-ray Absorption Spectroscopy of the Actinides-Outline

• Anomalous x-ray scattering • Edge jumps, pre-edge peaks, XANES, EXAFS • Edge shifts (following Conradson), electric dipole

selection rules, resolution issues, including lifetimes

• EXAFS-FT, electron scattering, Pu, UO2, UF4 (U L3 & F1s) and H20

• Pre-edge: Solomon, Clark & Minasian, 5f Covalent Bonding, UO2 and UF4

• Pu 5f Occupation, and relativistic and filling effects • Raman Scattering • RXES (Booth), Pu, UO2 & UF4, including

improvements in resolution and avoiding lifetime broadening

• Past: All U spectrum the same • HERFD (Kvashnina), All U spectra NOT the same • HERFD (Sokaras) UF4

Jim’s Eye View I apologize in advance for what I may have missed: This talk is based on a Jim’s Eye View of the world. ;-)


Anomalous X-ray Scattering

• Pb absorption edges

• J.E. Penner-Hahn

• Initially, in X-ray Diffraction and Scattering Experiments, the downward steps in intensity were considered anomalies

• Later, it became clear that they were transitions between electronic states

• X-ray Absorption • Steps are positive in absorption


Overview of an absorption edge (Flipping the peaks from neg to pos)

•Three main regions –Pre-Edge

–Near Edge or X-ray Absorption Near Edge Structure (XANES), also known as NEXAFS

–Extended X-ray Absorption Fine Structure (EXAFS)

• Figure by Munzarin at English Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=38941359


Let’s start in the middle Actinide Edge Jump Measurements

Conradson et al, Inorg. Chem 2003

Immense storehouse of XANES data shows edge shifts with oxidation state


Actinide Edge Jump Measurements Limited by broadening….


Step Fxn Resolution: The Width of a Guassian Step Fxn from 10% Height-90% Height is the same as the FWHM of a Gaussian Fxn.

These 10%-90% Edge Widths are about 10 eV….Not so good


The Actinide L3 XANES broadening: Both lifetime and instrumental

For the Act L3 XAS/XANES, an electron moves from the filled 2p states to the empty 6d states, in an electric dipole transition with Δl = ± 1.

However, we need to detect the transition. Usually, we can’t actually measure the transmission of the incoming x-rays and instead measure some emitted particle that follows the absorption. At lower photon energies, these are often electrons but in the hard x-ray regime, they tend to be x-ray emission, from the decay process to fill the core hole.

From the Heisenberg Uncertainty Principle, the short hole lifetime causes a finite energy width: lifetime broadening.


PuO2 L3 XANES: Lifetime & Instrumental Broadening

• High resolution Partial Fluorescence Yield (PFY) XAS and XES of Pu, as well as lower resolution Total Fluorescence Yield (TFY) XAS.

• TFY averages over the XES spectrum, thus getting a FWHM = 10 eV (Γ = 5 eV).

• J.G. Tobin, D.K. Shuh / Journal of Electron Spectroscopy

and Related Phenomena 205 (2015) 83–91


Actinide L3 XANES broadening: What does improved resolution do?

J. G. Tobin, et al., PRB 92, 035111 (2015).

By using the 6d UDOS from Ryzhkov’s calculations and varying the broadening, three separate XAS/XANES measurements can be explained (4d6d, 2p6d PFY XANES and 2p6d XANES)

Distinguish isoelectronic UO2 from UF4


We will return to the issue of edge shifts vs oxidation state but first, EXAFS…


Immense storehouse of XANES data shows edge

shifts with oxidation state

Utilizes a new, high-resolution, monochromator-based detection and a scattering based approach to lessen

life-time broadening

Breakthrough Experiment

on Pu, Booth et al, PNAS 2012


Extended X-ray Abs. Fine Structure (EXAFS) with Fourier Transform Analysis

• Sayers D.E, Stern E.A, Lytle, F.W., Phys. Rev. Lett. 1971, 27, 1204

• Stern, E. A.; Sayers, D. E.; Lytle, F. W. Phys. Rev. B 1975, 11, 4836

• Lee, P. A.; Pendry, J. B. Phys. Rev. B 1975, 11, 2795

• Ashley, C. A.; Doniach, S. Phys. Rev. B 1975, 11, 1279

Within a single scattering picture and using a Fourier Transform (FT) FT χ = Oscillations versus momentum (k) rNN = nearest neighbor distances


EXAFS-What’s going on….

• X-ray absorption fine structure (EXAFS, XAFS) is an oscillatory modulation in the X-ray absorption coefficient on the high-energy side of an absorption edge (Lee et al., 1981).

• Analysis of EXAFS can yield the numbers and types of atoms in the immediate environment of the absorbing atom, and accurate absorber-neighbor distances.

• smb.slac.stanford.edu/~ellis/Thesis/Chapter1.pdf


EXAFS-The underlying equation

• http://www.physics.mun.ca/~anand/teaching/p6900/CourseMaterials/talks/haneih.pdf


EXAFS-Multiple Scattering

•However, there’s a problem: electrons are highly interactive and prone to multiple scattering. •Single scattering models are very appealing but errors show up due to multiple scattering effects

• Figure from: https://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Spectroscopy/X-ray_Spectroscopy/EXAFS%3A_Theory


Multiple Scattering Theory

The FEFF Project at the University of Washington specializes in theoretical methods for spectroscopy. We offer a variety of condensed matter tools for modelling x-ray and electron spectroscopies and materials properties. These tools include the FEFF9 code, a popular and user-friendly program with a GUI that calculates XAS and EELS based on Green's functions theory.

Here is one commonly used solution to the multiple scattering problem.


EXAFS of Plutonium

• Ga K and Pu L3, edges, 10368.7 eV & 18059.0 eV

• L. E. Cox, R. Martinez, J. H. Nickel, S. D. Conradson, and P. G. Allen, PRB 51, JAN 1995


EXAFS isoelectronic cmpds distinguished

•UO2-highly ordered (fluorite/cubic)

•both nearest neighbor (O) and 2nd nearest neighbor (U) peaks

•UF4-monoclinic with two U sites

•nearest neighbor F only, U-U washes out jgt, jvsta 2015


EXAFS of UF4

• jgt, PRB 2015

• The Nearest Neighbor oscillations are obvious in both the U L3 (2p3/2) and the soft X-ray F K (1S) spectra, and supported by simulations.


One last EXAFS example: H20 vapor

• It is even possible to see the hydrogen atom EXAFS!

• O K (1s) Edge

• Osc from H • jgt PRL 2000


Now, back to the Prepeak: Cl K-edge XAS data provide a direct probe of the bonding to the spectator chloride ligand

EXP

Exp & Theory

Cl K edge

Solomon JACS 2005


The Cl K Edge Prepeak has subsequently been used to probe bonding in Actinide Cmpds

• Cl K edge

• Minasian, Clark et al, JACS 2012


A way to address 5f covalency

• Extended the work to transuranic chlorides (not shown) and oxides

• David Clark, Mumbai Talk


O K (1s) Edge

• Clark and the LANL Group has extended this approach to oxides, using the O 1s

• Other groups are also using the O1s XAS as a probe, e.g. Ward et al, Spectrochimica Acta 2017. (To the left)


Systematic Study of Covalency in UO2 and UF4 Using O 1s XAS; U 4d, 4f & 2p XAS & U 4d XES

jgt, et al., PRB (2015)

UF4 versus UO2

• First Peak (2p-5f) is diminished in both the F1s XAS and U 4d5/2 XES

• Second Peak (2p-6d) is strong but shifted in both the F1s XAS and U4d5/2 XES

• UO2 5f’s Covalent • UF4 5f’s Ionic At this point, the distinction between Prepeak and XANES has been completely lost, but this also leads us back to the crucially important issue of actinide 4d and 5d XAS…


Pu 5f Occupation

• Total fluorescence yield (TFY)

• Total electron yield (TEY)

• These changes are only understandable if the 5f electrons are highly relativistic, i.e. intermediate case, towards the jj limit.

• U & UO2 spectra are almost identical • The combined XAS and EELS data

indicated that n(5f) = 5 for Pu • PRL ‘03,PRL ‘04, PRB ‘05, JCMP 2008

The 4d XAS and Pu Occupation lead to two, new high resolution experiments…but first…


4d3/2 Peak Reduction

• This effect is caused by the combination of a large spin-orbit splitting in the 5f states and strong electric dipole selection rules.

• As the 5f5/2 fills, the intensity of the 4d3/2 drops, because there is no channel from the 4d3/2 into the 5f7/2.


No 5d Prepeak-pure jj n = 5

Uranium n5f = 3 [UO2 n5f = 2, same result]

(5d3/2)4(5d5/2)6(5f5/2)3(5f7/2)0 + hv

(5d3/2)4(5d5/2)5(5f5/2)4(5f7/2)0 + (5d3/2)4(5d5/2)5(5f5/2)3(5f7/2)1

(5d3/2)4(5d5/2)6(5f5/2)3(5f7/2)0 + hv (5d3/2)3(5d5/2)6(5f5/2)4(5f7/2)0

Plutonium n5f = 5 (5f5/2)6 is filled, therefore no coupling.

(5d3/2)4(5d5/2)6(5f5/2)5(5f7/2)0 + hv

(5d3/2)4(5d5/2)5(5f5/2)6(5f7/2)0 + (5d3/2)4(5d5/2)5(5f5/2)5(5f7/2)1

(5d3/2)4(5d5/2)6(5f5/2)5(5f7/2)0 + hv (5d3/2)3(5d5/2)6(5f5/2)6(5f7/2)0

Partially filled 5f’s and 5d holes cause prepeaks via angular momentum coupling and coulombic repulsion. Those at threshold (5d5/2 - 5f5/2) are most effective.


Two new and very exciting experiments, both based upon Raman Spectroscopy

Consider the example of Molecular Vibrational Spectroscopy Figure from “IR and Raman spectroscopy,” by Peter Hildebrandt

So what is Raman Spectroscopy?


Pu 5f Occupation : New Experiment Res X-ray Emission Spectroscopy

Based upon XANES data that shows edge shifts with oxidation state, e.g. Conradson et al, Inorg. Chem 2003

Utilizes a new, high-resolution, monochromator-based detection and a scattering based approach to lessen life-time broadening

Breakthrough Experiment on Pu, Booth et al, PNAS 2012

Multi-configurational picture for Pu with n(5f) near 5


New Hard-X-ray Spectrometer Conceptual Foundation

• BL 6-2 at SSRL

• Dimos Sokaras et al.


New Hard X-ray Spectrometer Instrumentation at BL 6-2, SSRL

• A seven-crystal Johann-type hard x-ray spectrometer at the Stanford Synchrotron Radiation Lightsource

• D. Sokaras et al., Review of Scientific Instruments 84, 053102 (2013)


Res X-ray Emission Spectroscopy Raman Scat. w/ better resolution than XAS Control Experiment with known systems

RXES Partial

Fluorescence Yield

• Ei (2p3/2) = 17,174 eV

• Ee(Lα1) = 13,614 eV

• ET = Ei – Ee

UF4

• nf = 2.046 2.0

• Г = 1.8 eV

• ionic, localized n = 2

UO2

• nf = 2.032 2.0

• Г = 2.7 eV

• n = 2, more covalent 5fs

UCd11 localized

n= 3

Ei

(eV)

jgt, PRB 2015


4d and 5d XAS lead us back to the problem of peak broadening

Tobin, JESRP 2014

•XPS shows big changes between U, UO2 ,UF4 and UO3

•Neither 5d nor 4d XAS show changes between U, UO2,UF4 & UO3

•Lifetime broadening is covering up changes in 4d and 5d XAS


High Energy Resolution Fluorescence Detection X-ray Absorption Spectroscopy

• Chemical State of Complex Uranium Oxides

• HERFD

• K. O. Kvashnina, S. M. Butorin, P. Martin, and P. Glatzel, PRL 111, 253002 (2013)


HERFD-U Oxides Kvashnina et al

• RIXS: Resonant Inelastic X-Ray Scattering

• A Raman Spectroscopy, to lessen lifetime broadening

• Fabulously successful

• The width of the peak to the left is a fraction of the width of the XANES Peak


HERFD/RIXS U Oxides

• Clearly, these U Oxides are easily distinguishable and in excellent agreement with theory

• A triumph!!!!

• Kvashnina, et al., PRL 111, 253002 (2013)


Meanwhile, back at SSRL, Sokaras et al are at it again….

• In a parallel effort, they are pursuing HERFD/RIXS

UF4 XAS vs U M5 HERFD

of Sokaras , Nowak &

Booth

Energy (eV)


HERFD Comparison: UF4 and UO2

• TFY/TEY are indistinguishable (earlier slide)

• UF4 has a pronounced shift, ~ 2 eV, to higher energy in XPS

• UF4 may have a similar shift in HERFD, but further work is necessary (The plotted shift may not be meaningful.)

• The L3 edge XANES show differences too

Tobin et al, PRB 2015

Ryzhkov UO2

UF4


Conclusions and Summary: There’s a lot going on out there & more to come!

• Anomalous x-ray scattering • Edge jumps, pre-edge peaks, XANES, EXAFS • Edge shifts (following Conradson), electric dipole selection rules,

resolution issues, including lifetimes • EXAFS-FT, electron scattering, Pu, UO2, UF4 (U L3 & F1s) and H20 • Pre-edge: Solomon, Clark & Minasian, 5f Covalent Bonding, UO2 and

UF4

• Pu 5f Occupation, and relativistic and filling effects • Raman Scattering • RXES (Booth), Pu, UO2 & UF4, including improvements in resolution

and avoiding lifetime broadening • Past: All U spectrum the same • HERFD (Kvashnina), All U spectra NOT the same • HERFD (Sokaras) UF4

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