![FIG. 1. (a) Corner perspective of the spectrometer in the XANES configuration. The SBCA and source are mechanically coupled to the center carriage. The twoaxis tilt is no longer utilized. The source and detector are at α = 40◦ (see Fig. 2). (b) CAD rendering of the helium box [removed from frame (a)] enclosing the x-ray beampath. The slots on the left and right faces are oriented at the height of the source and detector, while a rectangular cutout on the far face permits transit of x-rays to the SBCA. Each slot is covered by a polyimide film attached to the frame of the helium box.](/figures/fig-1-a-corner-perspective-of-the-spectrometer-in-the-xanes-17gnwc1r.webp)
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laboratory-based x-ray absorption fine structure and x-ray emission spectrometer for
analytical applications in materials chemistry research. Review of Scientific Instruments,
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Rev. Sci. Instrum. 90, 024106 (2019); https://doi.org/10.1063/1.5049383 90, 024106
© 2019 Author(s).
An improved laboratory-based x-ray
absorption fine structure and x-ray
emission spectrometer for analytical
applications in materials chemistry
research
Cite as: Rev. Sci. Instrum. 90, 024106 (2019); https://doi.org/10.1063/1.5049383
Submitted: 20 July 2018 . Accepted: 01 February 2019 . Published Online: 27 February 2019
Evan P. Jahrman , William M. Holden , Alexander S. Ditter , Devon R. Mortensen, Gerald T.
Seidler
, Timothy T. Fister, Stosh A. Kozimor, Louis F. J. Piper , Jatinkumar Rana, Neil C. Hyatt, and
Martin C. Stennett
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An improved laboratory-based x-ray absorption
fine structure and x-ray emission spectrometer
for analytical applications in materials chemistry
research
Cite as: Rev. Sci. Instrum. 90, 024106 (2019); doi: 10.1063/1.5049383
Submitted: 20 July 2018 • Accepted: 1 February 2019 •
Published Online: 27 February 2019
Evan P. Jahrman,
1
William M. Holden,
1
Alexander S. Ditter,
1,2
Devon R. Mortensen,
1,3
Gerald T. Seidler,
1,a)
Timothy T. Fister,
4
Stosh A. Kozimor,
2
Louis F. J. Piper,
5
Jatinkumar Rana,
5
Neil C. Hyatt,
6
and Martin C. Stennett
6
AFFILIATIONS
1
Physics Department, University of Washington, Seattle, Washington 98195-1560, USA
2
Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
3
easyXAFS LLC, Renton, Washington 98057, USA
4
Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, USA
5
Department of Physics, Binghamton University, Binghamton, New York 13902, USA
6
Materials Science and Engineering Department, The University of Sheffield, Mapping Street, Sheffield S1 3JD, United Kingdom
a)
seidler@uw.edu
ABSTRACT
X-ray absorption fine structure (XAFS) and x-ray emission spectroscopy (XES) are advanced x-ray spectroscopies that impact a
wide range of disciplines. However, unlike the majority of other spectroscopic methods, XAFS and XES are accompanied by an
unusual access model, wherein the dominant use of the technique is for premier research studies at world-class facilities, i.e.,
synchrotron x-ray light sources. In this paper, we report the design and performance of an improved XAFS and XES spectrom-
eter based on the general conceptual design of Seidler et al. [Rev. Sci. Instrum. 85, 113906 (2014)]. New developments include
reduced mechanical degrees of freedom, much-increased flux, and a wider Bragg angle range to enable extended x-ray absorp-
tion fine structure (EXAFS) measurement and analysis for the first time with this type of modern laboratory XAFS configuration.
This instrument enables a new class of routine applications that are incompatible with the mission and access model of the
synchrotron light sources. To illustrate this, we provide numerous examples of x-ray absorption near edge structure (XANES),
EXAFS, and XES results for a variety of problems and energy ranges. Highlights include XAFS and XES measurements of battery
electrode materials, EXAFS of Ni with full modeling of results to validate monochromator performance, valence-to-core XES for
3d transition metal compounds, and uranium XANES and XES for different oxidation states. Taken en masse, these results further
support the growing perspective that modern laboratory-based XAFS and XES have the potential to develop a new branch of
analytical chemistry.
Published under license by AIP Publishing. https://doi.org/10.1063/1.5049383
I. INTRODUCTION
X-ray absorption fine structure (XAFS) analysis is an espe-
cially capable and impactful tool for interrogating a mate-
rial’s local electronic and atomic structure. This element-
specific technique encompasses both the x-ray absorption
near edge structure (XANES), an acutely sensitive probe of
a compound’s oxidation state and molecular geometry, and
the extended x-ray absorption fine structure (EXAFS), which
is routinely used to extract multi-shell coordination numbers
and bond lengths. These techniques enable premier scientific
research campaigns in catalysis,
1,2
energy storage,
3,4
actinide
Rev. Sci. Instrum. 90, 024106 (2019); doi: 10.1063/1.5049383 90, 024106-1
Published under license by AIP Publishing
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chemistry,
5–7
heavy metal speciation in the environment,
8–10
etc. Likewise, the partner process, x-ray emission spec-
troscopy (XES), has been used to assess spin and ligand char-
acteristics, notably in critical discoveries of magnetic phase
transitions under geophysical conditions.
11,12
At present, XES
continues to emerge as an important measure of valence-
level (occupied) electronic state properties through improved
theoretical treatment of the valence-to-core (VTC) and core-
to-core (CTC) XES. However, as has been pointed out sev-
eral times in the modern history of XAFS and XES, and
most recently by Seidler,
13
these x-ray spectroscopies suf-
fer from an anomalous access model. In general, XAFS and
XES studies require access to synchrotron facilities with entry
requirements that limit more introductory, routine, or high-
throughput analytical studies that, by contrast, are common
for NMR, XPS, or optical spectroscopies where high-access
benchtop equipment is easily available.
Over the last several decades, the capabilities of lab-based
XAFS and XES instruments have rapidly grown. Researchers
now report spectrometers operating as low as the C K-edge
(284 eV)
14
using a laser-produced plasma source. The other
spectrometers probe the S and P K emission lines (∼2-2.5 keV)
using double crystal monochromators,
15–17
a dispersive Row-
land circle geometry,
18–21
and an instrument in the von Hamos
geometry.
22
A variety of von Hamos instruments exist which
are intended to operate in the ∼3-12 keV range needed for
studies of first row transition metals and lanthanides.
23–26
Many spectrometers operating in this range can be directly
integrated into synchrotron beamlines.
27
For similar energies,
a large number of XAFS spectrometers employing a Rowland
circle geometry exist.
28–33
Finally, higher energies, includ-
ing the Au Kβ (78 keV), are accessible via Laue-type spec-
trometers.
34
We focus here on the case of Rowland circle
geometries with a spherically bent crystal analyzer (SBCA),
which has been extensively developed by some of the present
authors.
13,35–40
The purpose of the present manuscript is to describe the
design and performance of what is our latest-generation of
improvements upon the first prototype instrument using an
SBCA.
13
These are embodied in two nearly identical spectrom-
eters, one at the University of Washington (UW) in Seattle
and the other at Los Alamos National Laboratory (LANL). Each
of these sites is more than 1000 km away from the nearest
synchrotron x-ray light source. The spectrometer improve-
ments include several simplifications to the monochromator
mechanical system, which decrease its operation from five
to only two motorized degrees of freedom and the selection
of a ten-fold higher power x-ray tube that retains the small
size and necessary anode characteristics to meet the needs of
laboratory XAFS and XES.
The manuscript continues as follows: first, in Sec. II,
we describe the new monochromator. Important highlights
include decreased mechanical complexity of the new design
and modification of the drive configuration to increase its
Bragg angle range while minimizing its air-absorption path
and overall footprint. Second, in Sec. III we present and dis-
cuss results for XANES and EXAFS of several materials reflect-
ing contemporary interest in materials chemistry and other
specialties. Examples include reference metal foils, battery
electrode laminates of several different compositions, a family
of reference Ce compounds, and uranium-rich materials. In all
cases, we find good agreement with prior synchrotron studies.
For the recorded EXAFS spectrum of the metal foil, we fur-
ther present a full Fourier-transform analysis using standard
methods and again find high quality results. Next, in Sec. IV
we present and discuss results for XES from a wide variety of
elements, chemical systems, and emission lines. This includes
both deep-shell emission lines and the VTC XES that provides
direct insight into chemical bonding. In Secs. III and IV, care
is taken to provide measurement times, thus serving as useful
benchmarks for assessing the feasibility of future studies using
SBCA-based laboratory monochromators.
II. EXPERIMENTAL
A. Monochromator design
Throughout the period between first publication
13
and
this work, several advances in the spectrometer design have
been made. Specifically, we have integrated a higher pow-
ered x-ray source, rotated and greatly elongated the source
and detector stages, implemented passive tracking of the
SBCA position (removing a motorized degree of freedom), and
enacted the tiltless optic alignment introduced by Mortensen
and Seidler
39
(removing two additional motorized degrees of
freedom). These changes were motivated by a focus on greater
count rates, instrument stability, ease-of-use, and achiev-
ing a wider useful energy range with each analyzer crystal
orientation.
An overview of the new spectrometer design is given in
Fig. 1. The approach uses linear translation stages to gener-
ate fine rotations (Bragg angle steps) and “steering bars” to
maintain alignment between the source, detector, and SBCA.
This design was based directly on our prototype system.
13
We
now summarize similarities and differences of the two new
instruments with respect to the prototype instrument.
First, the prototype system used a motorized transla-
tion stage underneath the SBCA to maintain its position
on the 1-m Rowland circle, while the source and detector
FIG. 1. (a) Corner perspective of the spectrometer in the XANES configuration.
The SBCA and source are mechanically coupled to the center carriage. The two-
axis tilt is no longer utilized. The source and detector are at α = 40
◦
(see Fig. 2).
(b) CAD rendering of the helium box [removed from frame (a)] enclosing the x-ray
beampath. The slots on the left and right faces are oriented at the height of the
source and detector, while a rectangular cutout on the far face permits transit of
x-rays to the SBCA. Each slot is covered by a polyimide film attached to the frame
of the helium box.
Rev. Sci. Instrum. 90, 024106 (2019); doi: 10.1063/1.5049383 90, 024106-2
Published under license by AIP Publishing
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positions (and hence Bragg angles) were scanned. In the
present instrumentation, a passive linear translation stage
with two carriages is used; one carriage for the SBCA and
another carriage for a pin located at the moving center of the
Rowland circle. Coupling bars with lengths equal to the radius
of the Rowland circle constrain the Rowland-center pin to be
the correct distance (0.5 m) from pins underneath both the
SBCA and the source location. This direct mechanical cou-
pling provides exceptional scan-to-scan reproducibility and
decreases instrument complexity by removing one motorized
degree of freedom.
Second, the source and detector stages have been rotated
and made much longer than in the earlier system. The longer
travel range allows access to Bragg angles between 55
◦
and
85
◦
, a considerable improvement over the prototype that
allows a much wider energy range for each crystal orien-
tation of SBCA. This change decreases the total number of
SBCA optics required for XAFS and XES analysis. Moreover,
it extends the utility of the spectrometer beyond XANES,
enabling EXAFS studies for several elements. The stage rota-
tion requires some careful comment. The resulting stage
geometry is shown in Fig. 1, and the rotation parameter α is
defined in Fig. 2(a). The issue that motivates the rotation of the
stages is the desire to minimize the linear travel of the SBCA
needed to maintain its position on the (traveling) 1-m Row-
land circle. Long SBCA travel is not mechanically onerous, but
clearance is required with respect to the helium box [Fig. 1(b)]
enclosing the beampath to reduce air absorption. When the
SBCA has a long travel, the helium box must be made shorter
which results in higher air absorption for most operations. To
address this problem, a suitable value of α can be deduced
from geometric considerations. As the source and detector are
swept outward to smaller Bragg angles, the SBCA is necessar-
ily displaced to ensure the source and detector remain on the
Rowland circle. This displacement d(θ
B
) is given by
d
(
θ
B
)
= −R ∗ sec[α] ∗ (cos[ θ
o
− α] + cos[α + 2θ
B
]), (1)
where R is the radius of the Rowland circle, θ
B
is the Bragg
angle, and the displacement is measured relative to the posi-
tion of the SBCA when θ
B
= 85
◦
, this value is denoted above
as θ
o
. In
Fig. 2(b), Eq. (1) is plotted as a function of θ
B
for var-
ious values of α. It is apparent that translation of the SBCA,
and consequently attenuation due to air outside of a fixed
helium enclosure, can be minimized by an appropriate choice
of α. This translation is minimized when the SBCA’s travel is
symmetric across the angle range, which can be enforced by
choosing α to be equal to 180
◦
minus twice the midpoint of the
angle range. For a θ
B
range of 85
◦
–55
◦
the SBCA’s displacement
is minimized when α = 40
◦
, as is utilized in Fig. 1. Moreover, an
additional benefit of the stage rotations is a smaller instrument
footprint.
Third, the present design discontinues the traditional
two-axis tilt alignment of the SBCA in favor of orienting
the crystal miscut into the plane of the source and detec-
tor and enforcing a constant angular offset of the detec-
tor, as described by Mortensen and Seidler.
39
This removes
two motorized degrees of freedom and also enables easy,
reproducible exchange of different SBCAs for different energy
FIG. 2. (a) Illustration depicting the parameter α and θ
B
. The SBCA resides at the
bottom of the Rowland circle, while the carriage coupling the SBCA location and
the source as represented by the hollow dot is at the center of the Rowland circle.
The diagonal line represents the travel range of the source with dots at its end
points. (b) The magnitude of the SBCA’s displacement from its location, d(θ
B
), at
θ
B
= 85
◦
is plotted as a function of θ
B
for various values of α.
ranges. Here, SBCAs are aligned by performing repeated
detector scans at different rotations of the optic about its
natural cylindrical axis. This fast process gives a permanent
alignment orientation. See Fig. 3 for representative calibra-
tion scans. Note that the highest count rates are generally
observed when the crystal miscut is oriented into the Row-
land plane, as the SBCA is rotated in either direction away
from this orientation, the centroid of the corresponding scans
moves in the same direction away from the peak at optimal
orientation.
Fourth, from a practical standpoint, the primary benefit
of a high-flux source is shorter acquisition times and thus
higher potential instrument throughput. Moreover, greater
flux can broaden an instrument’s limit-of-detection, thus
enabling studies of particularly dilute samples or weak tran-
sition lines. Nonetheless, there exist several points of con-
cern when utilizing a high-powered tube source. Specifically,
Rev. Sci. Instrum. 90, 024106 (2019); doi: 10.1063/1.5049383 90, 024106-3
Published under license by AIP Publishing