Rev. Sci. Instrum. 89 , 113111 (2018); https://doi.org/10.1063/1.5035171 89 , 113111
© 2018 Author(s).
A laboratory spectrometer for high
throughput X-ray emission spectroscopy in
catalysis research
Cite as: Rev. Sci. Instrum. 89 , 113111 (2018); https://doi.org/10.1063/1.5035171
Submitted: 13 April 2018 . Accepted: 24 October 2018 . Published Online: 15 November 2018
Wolfgang Malzer, Daniel Grötzsch, Richard Gnewkow, Christopher Schlesiger, Fabian Kowalewski,
Benjamin Van Kuiken , Serena DeBeer, and Birgit Kanngießer
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REVIEW OF SCIENTIFIC INSTR UMENTS 89 , 113111 (2018)
A laborator y spectromet er for high throughput X -ray emission
spectroscop y in catalysis research
W olfgang Malzer, 1 Daniel Gr ¨
otzsch, 1 Richard Gnewk ow, 1 Christ opher Schlesiger, 1
F abian K ow alewski, 2 Benjamin V an K uiken, 2 Serena DeBeer, 2 and Bir git Kanngießer 1
1 Institute for Optic and Atomic Physics, T ec hnische Universit ¨
at Berlin, Har denber gstr . 36, 10623 Berlin, Germany
2 Max Planc k Institute for Chemical Ener gy Con version, Stiftstr . 34-36, 45470 M ¨
ulheim an der Ruhr , Germany
(Recei ved 13 April 2018; accepted 24 October 2018; published online 15 No vember 2018)
W e hav e built a laboratory spectrometer for X-ray emission spectroscopy . The instrument is employed
in catalysis research. The ke y component is a von Hamos full c ylinder optic with Highly Annealed
Pyrolytic Graphite (HAPG) as a dispersi ve element. W ith this v ery efficient optic, the spectrometer
subtends an ef fectiv e solid angle of detection of around 1 msr , allo wing for the analysis of dilute
samples. The resolving po wer of the spectrometer is approximately E/ ∆ E = 4000, with an energy range
of ∼ 2.3 keV –10 k eV . The instrument and its characteristics are described herein. Further , a comparison
with a prototype spectrometer , based on the same principle, sho ws the substantial improv ement in the
spectral resolution and ener gy range for the present setup. The paper concludes with a discussion of
sample handling. A compilation of HAPG fundamentals and related publications are gi ven in a brief
Appendix. Published by AIP Publishing. https://doi.or g/10.1063/1.5035171
INTRODUCTION
In X-ray emission spectroscopy (XES), X-ray line spectra
are measured with a spectral resolution suf ficient to analyze the
impact of the chemical en vironment on the X-ray line ener gy
and on branching ratios. T o our knowledge, the first XES exper -
iments were published by Lindh and Lundquist in 1924. 1 In
these early studies, the authors utilized the electron beam of
an X-ray tube to excite core electrons and obtain the K β line
spectra of sulfur and other elements. Three years later , Coster
and Druyveste yn performed the first experiments using photon
excitation. 2 Their w ork demonstrated that the electron beams
produce artifacts, 2 thus moti v ating the use of X-ray photons
for creating the core hole. Subsequent e xperiments were car -
ried out with commercial X-ray spectrometers, 3 as well as with
high-resolution spectrometers. 4 While these early studies pro-
vided fundamental insights into the electronic configuration
of small molecules, XES only came into broader use with the
a vailability of high intensity X-ray beams at synchrotron radia-
tion facilities, which enabled the measurement of (chemically)
dilute samples. 5 In addition to the experimental adv ances, it
is also the progress in quantum chemical computations, which
makes XES an intriguing tool for the study of the electronic
structure of chemical compounds. 6 , 7
While synchrotron radiation experiments provide a unique
sensiti vity and flexibility , the access to it is limited. The moti-
v ation of this work is to ov ercome this limitation and to better
exploit the potential XES of fers for research in chemistry . The
goal was to b uild a laboratory based spectrometer , which is
capable of high-throughput XES analysis of samples that are
rele vant in catalysis research.
During the last decade, v arious X-ray analyzers for syn-
chrotron applications were b uilt. In order to preserve high
resolving po wer and to increase the solid angle of detection,
multi-crystal analyzer setups were de veloped. These include
setups based on the Johann geometry , which were built by
Sokaras et al. 8 and Kleymeno v et al. , 9 as well as von Hamos
type setups b uilt by Hayashi et al. 10 and Alonso-Mori et al. 11
In addition, laboratory spectrometers for XES, which are based
on spherically bent crystals, were de veloped by Seidler et al. 12
and Holden et al. 13
The XES spectrometer , which is presented in this paper , is
based on the de velopments published pre viously by Anklamm
et al. 14 As an X-ray tube is used for excitation, which is
much less brilliant than synchrotron radiation, the analyzer
part of the spectrometer must be e xtremely ef ficient to allow
for the detection of the weak v alence-to-core lines ev en in
dilute samples.
The laboratory XES spectrometer is based on the v on
Hamos principle, and the entire spectrum is taken without mov-
ing any spectrometer components. In order to achie v e the high
ef ficiency required for XES with laboratory sources, a full 360 ◦
collection geometry is utilized. The X-ray lines are mapped
as rings onto the CCD, which is used as a position sensiti ve
detector . T o further maximize the sensitivity , a mosaic crys-
tal with high integral reflecti vity , Highly Annealed Pyrolytic
Graphite (HAPG), was used as a dispersi ve element. The effec-
ti ve solid angle of such a ring optic has a magnitude of around
1 msr .
The first setup we de veloped used a 100 W X-ray tube with
a poly-capillary concentrator for the excitation of the sample
and an X-ray CCD for detection. The spectral resolving po wer ,
we achiev ed with the prototype setup, was approximately E/ ∆ E
= 2000, and the ef fectiv e solid angle (reflecti vity × solid angle)
cov ered by the analyzer optic is in the range of a few msr . The
instrument is capable of measuring good quality XES spectra
on dilute samples ov er a period of se veral hours.
In comparison with this first setup, major improv ements
could be achie ved. The spectral resolving po wer of the instru-
ment presented in this paper could be increased significantly
from E/ ∆ E = 2000 to E/ ∆ E = 4000. The loss in the ef fectiv e
solid angle of detection connected with the higher resolving
0034-6748/2018/89(11)/113111/8/ $ 30.00 89 , 113111-1 Published by AIP Publishing.

113111-2 Malzer et al. Re v . Sci. Instrum. 89 , 113111 (2018)
po wer was compensated by a more po werful excitation through
a Ga-jet X-ray source. The range of elements, which can be
analyzed, was also e xtended. The first setup was capable of
measuring the K-lines of elements with atomic numbers higher
than Ca. W ith the new setup, it is possible to measure the
K-lines of sulfur -containing compounds.
O VER VIEW OF THE INSTRUMENT
Figure 1 sho ws a graphical view of the XES spectrometer .
The lar ge v acuum vessel on the right side houses the X-ray
analyzer . The ring optic and the X-ray CCD are mounted on
a 3.5 m long rail system. At the lar gest distance possible, the
Zn K β lines at ∼ 9.5 keV can be measured in the first order of
reflection. W ith the ring and the CCD positioned as close to
the sample as possible, sulfur emission lines at 2.3 keV can be
measured.
The sample is excited by a Ga-jet X-ray source manufac-
tured by Excillum ( www .e xcillum.com ), which is operated at a
po wer of 250 W . As the diameter of the analyzer housing is 800
mm, the radiation has to be transferred ov er a distance of almost
500 mm from the X-ray source to the sample. This is achie ved
with a polycapillary X-ray lens manufactured by the Institute
for Scientific Instruments (IFG, www .ifg-adlershof.de ). The
X-rays are concentrated onto a spot of ∼ 30 µ m in diameter .
The FWHM of the focal spot was measured by a knife edge
scan. The intensity gain, determined as the ratio of count rates
measured through a pinhole of 10 µ m diameter , is specified by
the manufacturer to be slightly abo ve 20 000 within the ener gy
range of 5 keV –20 keV .
A directly detecting deep depletion back illuminated
X-ray CCD (Princeton Instruments) serves as a detector . The
detector area is 1 in. × 1 in. with 1300 × 1340 20 µ m pix els.
The samples are loaded into the instrument through the
glov e box on the left side. Inside the glov e box, they are
fixed onto the sample holder . A bayonet clutch serves as the
mount for the sample holder to the finger of the cryo-cooling
unit (Adv anced Research Systems, Cryoandmore). After the
FIG. 1. Schematic view of the XES spectrometer . On the right hand-side,
the ring, which is coated with the HAPG crystal, is displayed. T ogether
with the CCD, it is mounted on a rail system which allo ws Bragg angles
to be selected for an energy range of 2.3 k eV –10 keV . The load lock can
be separated by a v alve from the analyzer vessel. As the load lock is inside
the glov e box, sample handling without exposure to oxygen or humidity is
possible.
FIG. 2. K α spectrum of sulfur . The S K α spectrum of FeS was taken in tw o
steps as the energy bandwidth at 2300 eV is 20 eV . The measurements took
1 h for the main lines and 5 h for the satellite lines abov e 2317 eV .
v acuum load lock is ev acuated, the sample can be transferred
to the main v acuum chamber and mov ed into the focus of the
polycapillary X-ray lens.
By performing the measurements in v acuum, the accessi-
ble ener gy range of the present instrument is greatly e xtended
relati ve to that of Anklamm et al. The prototype setup has an
entrance windo w , which separates the sample, measured in air ,
from the analyzer operated in v acuum. While the crystal and
the detector can be positioned for ener gies down to 2.5 k eV ,
the entrance windo w increasingly masks parts of the crystal
ring beginning at the Ca K-alpha lines. In combination with
absorption due to the entrance windo w , the spectrometer’ s sen-
siti vity drops off drastically to ward lo wer ener gies and renders
it useless belo w ∼ 4 keV .
By contrast, with the sample in the same v acuum vessel
as the analyzer , the measurement of emission lines down to
the sulfur K-alpha lines is feasible. Figure 2 depicts an S K α
spectrum of FeS. The measurement was carried out in tw o
steps with 1 h for the main lines and 5 h for the satellite lines
in the ener gy range from 2317 eV to 2326 eV .
SPECTROMETER CHARA CTERISTICS
A major design goal of the present spectrometer de velop-
ment was to increase the spectral resolving po wer relati ve to
the pre vious instrument. For this reason, the geometry of the
ring was modified. Namely , its diameter was doubled, from
300 mm to 600 mm, and the thickness of the HAPG crystal
layer was halv ed from 40 µ m to 20 µ m. An optic of this size
presents a real manufacturing challenge as the mold must con-
sist of glass and the surface must be polished to optical quality
without introducing shape errors. If these conditions were both
met, the mosaic spread of the HAPG would not be as lo w as
required. In a cooperation of the optic manufacturer Optigraph
( www .optigraph.eu ) and TU Berlin, the mold for the ring optic
could be manufactured in one piece.
Besides the thickness of the crystal, its mosaic spread
is crucial for a high resolving po wer and low peak tailing.
W e determined the mosaic spread of the HAPG crystal in

113111-3 Malzer et al. Re v . Sci. Instrum. 89 , 113111 (2018)
FIG. 3. Determination of the mosaic spread. The figure
outlines the determination of the mosaic spread follo w-
ing Jarrott et al. 17 For the characterization of the mosaic
spread, only 20 ◦ of the crystal ring is illuminated. A cus-
tomized sample holder is used to block the remaining
340 ◦ of the optic. The CCD is positioned at the focal point
of the emission line, which is used for the measurement.
The image of this line is extended to ward the direction of
dispersion and perpendicular to it. The broadening per-
pendicular to the direction of dispersion is mainly due
to the mosaic distribution of the crystallites. The y reflect
X-rays within a small segment of a cone, indicated by
dotted lines.
18 sections of 20 ◦ each. For that purpose, we used an Fe-
tar get inside a customized sample holder . This holder shielded
most of the ring optic, allo wing for the irradiation of only 20 ◦
sections of the optic. The setup for this measurement resem-
bles the standard v on Hamos geometry . The X-ray CCD was
positioned in the focal plane of the Fe K α 1 line.
Figure 3 illustrates the principle of the measurement,
which was also used by Jarrott et al. 17 The CCD image of
the X-ray line sho ws broadening not only in the direction of
dispersion b ut also perpendicularly to it in the sagittal direc-
tion. The sagittal profile is dominated by a contrib ution from
the finite source size and an additional broadening, which is
directly caused by the mosaic profile. Therefore, the mosaic
spread can be determined from the sagittal profile of X-ray
emission lines. The mosaic spread of the crystal was deter -
mined to be 0.06 ◦ , which is the optimum achie v able according
to our experience.
Based on the measurement of the mosaicity , the source
size (30 µ m), the pix el size of the detector (20 µ m), and an
intrinsic broadening of 14 arc sec, 15 we determined the spectral
resolving power . The result of the calculation is shown in Fig. 4
together with the result of an experimental determination,
which follo wed the procedure described by Anklamm. 14 , 16
The spectral broadening of this spectrometer is dominated
by the intrinsic broadening of the crystal. Thus, the spectral
resolving po wer is close to the maximum achiev able. At this
point, we also note that the actual resolution strongly depends
on the sample preparation. In certain cases, the thickness of
the sample may dominate the resolution, as is discussed in
detail later in this report. The resolution also v aries with the
ring position on the detector , as sho wn in a previous paper by
Anklamm et al. 14
Experimentally determined v alues and calculated values
de viate by up to 5% from each other . As mentioned above, the
v ariation of attenuation length affects the actual source size
and thus the spectral resolution. The estimate of the resolution
does not take into account these v ariations. Further , errors of
literature data for the core hole lifetime broadening and the
simplified X-ray line model consisting of only the two major
lines K α 1 and K α 2 add to the uncertainty of the e xperimentally
determined spectral broadening.
A comparison of spectra taken with the first setup and the
ne w one demonstrates the improv ement (Fig. 5 ). W e note that
both the resolving po wer (in terms of FWHM of the peaks)
and also the tailing are reduced strikingly . As discussed in
detail in Anklamm et al. , 14 one of the consequences of the
crystal mosaicity is an asymmetric peak profile. The pre vious
HAPG ring optic sho ws a pronounced tailing on one side,
which hampers the e valuation of weak emission lines located
next to strong ones. The reduction in the mosaic spread from
pre viously 0.1 ◦ to 0.06 ◦ reduces the tailing, and the K β ’ in the
FeS spectrum sho wn in the right panel of Fig. 5 is clearly better
resolved from the main line as compared to the measurements
obtained with the prototype instrument.
As a consequence of optimizing the resolving po wer , the
optic’ s effecti ve solid angle of detection is lo wer than that of the
FIG. 4. Resolving power . The resolving po wer of the XES spectrometer is
approximately E/ ∆ E = 4000 ov er the entire range of energy . The calculated
v alues include the broadening by the mosaic crystal, by the source size and
by the pixel size of the detector . Measured resolving po wers were determined
using metal foils. Dilute samples may sho w noticeable penetration ef fects,
which worsen the spectral resolution.

113111-4 Malzer et al. Re v . Sci. Instrum. 89 , 113111 (2018)
FIG. 5. Comparison between the current setup and the prototype setup reported in Ref. 4 . The graph on the left-side demonstrates the improv ed resolving po wer
and the reduction in the peak tailing by a comparison of Cu K α spectra. This improv ement is also obvious in the example sho wn on the right-side, which depicts
the K β multiplet of FeS. The K- β ’ line at the lo w energy side of the main line is much better resolved.
first setup. The main reason is the lower thickness of the crystal
layer , which reduces the integral reflecti vity by approximately
a factor of tw o. Figure 6 compares the ef fecti ve solid angle of
detection for both optics. W e note that the ef fectiv e solid angle
of the ring optic is reported as the product of the solid angle
subtended and the integral reflecti vity of the crystal.
This loss in the solid angle of detection is more than com-
pensated by the use of a more po werful X-ray source. W e used
a Ga-jet X-ray source, which is operated at 250 W . The adv an-
tages of the Ga-Jet source include the high brilliance, as well as
the use of Ga as a tar get, which characteristic radiation excites
first ro w transition metals up to Cu, and ev en Zn with Ga K β .
The radiation is transferred to the tar get by using a 500 mm
long polycapillary X-ray lens with a focal spot size of around
30 µ m.
The ener gy band width of the instrument depends on the
meridional length of the crystal and on the size of the detector .
FIG. 6. Effecti ve solid angle of the optic and spectrometer . The plot compares
the ef fective solid angle for the center ener gy of the prototype spectrometer
with the ne w one. For the ne w spectrometer , the effecti ve solid angle of the
optic is sho wn. It represents the optics capability to reflect photons emitted by
the source onto the detector . The effecti ve solid angle of the entire spectrometer
additionally takes into account the detector’ s quantum ef ficiency . The effecti ve
solid angle of the prototype spectrometer is larger because the crystal is 40 µ m
thick in comparison with 20 µ m for the new one. Belo w 5 keV , its effecti ve
solid angle decreases due to absorption in the entrance windo w .
Figure 7 depicts both limits. In the ener gy range from 2.3 keV
to 6 keV , the size of the X-ray CCD restricts the ener gy range,
which increases linearly from ∼ 20 eV to ∼ 110 eV . Abov e
6 keV , the crystal length confines the bandwidth, which only
slightly increases to ∼ 125 eV at 10 keV .
CCD IMA GE EV ALU A TION
The X-ray emission lines appear as rings on the CCD, as
sho wn in Fig. 8 . The left image displays the Cu K-lines of
metallic copper , the K α 1 at 8046.3 eV , and the weaker K α 2
at 8026.7 eV . The right image shows the tw o Cl K α -lines for
KCl at 2622 eV and 2620 eV , respectiv ely . While the Cu-
lines produce well-shaped homogeneous rings, the Cl-rings
are se verely distorted. These distortions are an implication of
the lar ge take of f aperture of ca. 90 ◦ . The incidence angle of
the beam to ward the sample surface must be shallo w , and its
actual footprint is around 5 times lar ger in the plane of beam
and spectrometer axis compared to the v ertical axis. The result
FIG. 7. Energy bandwidth. The bandwidth limit imposed by the size of the
detector (1 in. × 1 in.) is plotted in green. The bandwidth restriction due
to meridional length of the crystal (20 mm) is sho wn by the blue line. In
the energy range between 2.3 k eV and 6 keV , the bandwidth is limited by
the size of the detector . Above 6 k eV , the crystal length confines the energy
range.

113111-5 Malzer et al. Re v . Sci. Instrum. 89 , 113111 (2018)
FIG. 8. CCD image of Cu and Cl K α lines. The chlorine spectrum of KCl
was taken in a pre-focus geometry , where the radius of the ring increases
with increasing energy . The copper spectrum was measured in a post-focus
geometry , where this relation in in verted (see the work of Anklamm et al. 14
for details). The weaker intensity at 11 o’clock, which the two spectra ha ve
in common, is due to a HAPG segment with slightly lo wer reflecti vity . In
comparison with the otherwise perfect circles of the X-ray image for the K α
lines of metallic copper in the left panel, the intensity image for chlorine sho ws
self-absorption ef fects. With the lar ger take of f cone (indicated in red), the take
of f angle considerably v aries, and the ef fects of self-absorption become sev ere.
This asymmetry required a re vision of the image ev aluation as described in
the respecti ve section.
is the broadening of the ring in that plane. At the same time,
the take of f angle v aries between an almost grazing angle on
one side and almost perpendicular on the other side, causing
a considerable dif ference in the count rate. These distortions
made the automated CCD image e valuation as described belo w
fail.
The CCD image e valuation consists of three steps: deter -
mine the center of the rings, sum the e vents along radius
channels, and con vert the radius channels to ener gy channels. 4
The first step failed for the e v aluation of measurements in the
lo w energy range (e.g., S and Cl XES). The center of the ring
was determined by a fit with a circle using the most intense
pixels, b ut the strong asymmetries caused the center to be com-
pletely off. The procedure we follo w no w is more complex. For
a gi ven center , the spectrum is determined. The FWHM of the
most intense peak is used as the criterion for the optimization.
This ne w procedure is robust and reliable.
Further , the ev ent detection was improv ed. T o increase the
signal to noise ratio for dilute samples, the CCD is operated
in the single photon counting mode. When an X-ray photon
is absorbed in the wafer of the detector , electron-hole pairs
are created. The amount of char ge created is proportional to
the ener gy of the interacting photon. This allo ws energy dis-
crimination with a CCD and can be used to suppress dark
and readout noise, as well as e vents caused by higher order
contrib utions or background radiation.
Depending on the pixel and char ge cloud size, the total
char ge created by a single photon e vent can spread ov er mul-
tiple pixels. W e improv ed the e vent detection for these split
e vents using a pix el clustering approach with a spectral res-
olution of around 190 eV for Fe K α . W e added a detection
of pile-up e vents, which no w are attributed to the X-ray lines
under in vestigation. These impro vements increased the yield
of registered photons to almost 100%.
S AMPLE PREP ARA TION AND EXPERIMENT AL
PRA CTICE
One of the research approaches, which the XES spec-
trometer is used for , is comparisons of the spectra of series
of compounds where the chemical en vironment of the ele-
ment in vestigated is altered systematically and purposefully .
The precision of the ener gy scale is crucial for this type of
in vestigation.
The relation between photon ener gy and the ring on the
detector depends on the Bragg angle, the radius of the X-ray
optic, and the distance between source and the center of the
ring image on the detector . The respecti ve equation is gi ven in
Anklamm et al. 14 In an initial ener gy calibration, the distance
between the source and the detector is determined by means
of a reference sample with kno wn emission line energies. The
spectrometer can be tuned to any ener gy with mechanical pre-
cision. As sho wn subsequently , the best precision requires the
use of a reference sample before the analysis of unkno wn sam-
ples. The spectrometer allo ws a temporary redefinition of the
source to sample distance.
As outlined abov e, the energy of an X-ray line is related
to the diameter of its image on the CCD, and the distance
between the source and the CCD is the critical property for
the precision of the ener gy scale. Sample misalignment per -
pendicular to the optical axis of the spectrometer is much less
critical. W e in vestigate the stability and precision of the spec-
trometer ener gy scale for three modes of operation. First, for
comparisons, where only the sample is changed and neither
the CCD nor the crystal is mo ved between measurements. W e
aligned the test samples 10 times each and determined the
standard de viation of the peak positions. For sample align-
ment, the center of the ring on the CCD is determined with
a short measurement after bringing the sample into the beam.
As the coordinates of the ring center for an aligned sample
are kno wn from the calibration, the deviation can be compen-
sated. In most cases, the sample is aligned correctly after one
step.
Second, we mov ed the CCD, which sometimes is useful,
because one can inspect neighboring energy ranges. As the
positioning of the CCD stage will dominate the uncertainty ,
the sample was not re-aligned for each of the measurements.
Finally , the energy scale w as not determined by a refer -
ence measurement, b ut the spectrometer was simply set to the
ener gy according to the initial ener gy calibration. CCD and
crystal were mov ed between the test measurements in order to
e valuate the reproducibility of motor mo vements as a possible
source of error .
For the first mode of operation, the peak position had a
standard de viation of around 20 meV for Cu, Cr , and K. W e,
ho wev er , note that this precision cannot be simply assumed
for any e xperiment. Changing the model for peak fitting typi-
cally results in v ariations in the resulting line energies, which
may be lar ger than the instrumental v ariations. The asym-
metric spectrometer response hampers spectrum fitting. If we
compare dif ferent substances, the uncertainty of line energies

113111-6 Malzer et al. Re v . Sci. Instrum. 89 , 113111 (2018)
will be higher than the de viations we obtained by the repeated
measurement of the same substance.
The standard de viation for the second mode of measure-
ment, the CCD was mo ved in between, ranges from 40 meV
to 80 meV .
If the spectrometer is just mov ed to a ne w ener gy without
recalibration with a reference measurement, the ener gy scale
is shifted up to 1 eV . The precision for moving all spectrometer
components has a standard de viation of 150 meV .
A final important practical consideration is the nature of
the sample itself. For po wder samples, hard X-ray measure-
ments are commonly carried out by filling a metal spacer with
sample po wder and sealing it with transparent tape. T ypical
sample thicknesses are on the order of ∼ 1 mm. For our instru-
ment, the thickness of a dilute sample has a pronounced ef fect
on the measurement resolution. This is due to the fact that a
true sample is not a point source. Hence, photons emitted from
the front and the back of the sample yield dif ferent distances
between the source and the optic and are thus imaged at dif fer-
ent points on the detector . In order to in vestigate the ef fects of
sample thickness, a series of iron oxide (Fe 2 O 3 ) samples was
prepared with increasing dilution in BN. All samples were
packed in 1 mm sample holders and measured at an angle of
45 ◦ with respect to the incident beam. Figure 9(a) sho ws Fe
K α 1 for the series of Fe oxide samples, and T able I compares
the attenuation length for each sample with the FWHM of the
K α 1 and the count rate at the maximum of the K α 1 . Clearly
there is a strong dependence of the spectral width on the atten-
uation length since the FWHM of the K α 1 nearly doubles as
the attenuation length increases from 9 to 606 µ m.
The dependency of the spectral width on the attenuation
length is potentially problematic when the sample is inherently
dilute. For example, one may be interested in series of catalysts
where a dopant appears in dif ferent concentrations. This could
result in v arying spectral widths and complicate the interpre-
tation of the underlying chemistry . There are sev eral possible
FIG. 9. Impact of sample characteristics on XES resolution. The dependence
of the Fe K α 1,2 spectrum of Fe 2 O 3 when diluted in increasing amounts of
BN (a). Mass percents of Fe 2 O 3 and the attenuation length of the X-ray beam
determined from atomic scattering factors are listed. Comparison of thick
(blue line) and thin (dashed red line) samples for Fe 2 O 3 (b) and Fe(TPP) (c).
T ABLE I. Dependence of sample concentration on attenuation length, spec-
tral width, and count rate.
Attenuation FWHM of Count
Sample length ( µ m) K α 1 (eV) rate (10 6 /s)
100% Fe 2 O 3 9 4.83 2.5
25% Fe 2 O 3 :BN 61 5.47 1.5
6.25% Fe 2 O 3 :BN 250 6.83 0.66
1.56% Fe 2 O 3 :BN 606 7.87 0.21
Fe(TPP) . . . a 6.26 0 .56
Thin Fe 2 O 3 . . . 4.73 0.55
Thin Fe(TPP) . . . 4.52 0.01
a Despite being a pure complex, the density of Fe(TPP) is not easily determined.
corrections for this issue. First, one could dilute all samples
to obtain samples of equal concentration. This would result
in comparable albeit broadened spectra. Second, thin samples
can be employed. Figures 9(b) and 9(c) compare the results
of using “thick” and “thin” samples in Fe 2 O 3 and an inher -
ently dilute sample, an iron tetraphenyl porph yrin, denoted
Fe(TPP), respecti vely . Thin samples were prepared by spread-
ing a thin layer of the sample po wder on Kapton tape. In the
case of Fe 2 O 3 , the thick and thin samples are nearly identical,
indicating that no substantial decrease in resolution is incurred
for concentrated samples such as transition metal oxides. On
the other hand, the dilute molecular sample sho ws a signifi-
cant dif ference. The 1 mm thick sample of Fe(TPP) has a K α 1
line width of 6.26 eV , but the thin sample e xhibits a 4.52 eV
width. Thus, the Fe(TPP) examples sho w a route for recov-
ering the maximum instrument resolution for dilute systems
through thoughtful sample preparation.
While preparing thin samples assures that one obtains the
highest possible experimental resolution, thinner samples also
result in decreased count rates. For a pure Fe 2 O 3 , one achie ves
2 × 10 6 counts per second (cps) at the maximum of the K α 1 . For
dilute samples of iron oxide and for molecular complex es with
lar ge ligand systems, the count rate is an order of magnitude
less. Similarly , there is about an order of magnitude decrease
in going from a thick sample to a thin sample in the systems we
ha ve examined. F or all samples, virtually noiseless K α lines
can be obtained relati vely quickly . On the other hand, if K β
v alence-to-core spectra are desired, the counts are predicted
to be reduced by a factor of 10 3 relati v e to the K α 1 line. Con-
sequently , there is a trade-off between the spectral resolution
provided by thin samples and number of counts pro vided by
thick concentrated samples. As a matter of practicality , mea-
suring only po wder spread on tape is not feasible for K β VtC
measurements due to the lo w counts. For e xample, the spec-
trum of Fe(TPP) spread on tape sho ws no well-resolved VtC
feature after 7 h.
In summary , sample preparation must balance feasibility
and spectral resolution, and the specific sample preparation
strategy will depend on the chemical system of interest. Many
heterogeneous catalysts are based on relati vely simple concen-
trated complex es, such as transition metal oxides. Both thick
and thin samples are likely suitable due to high signal le vels
and short X-ray attenuation lengths. For more dilute samples,
such as molecular homogeneous catalysts, optimal sample
conditions must be chosen balanced between the resolution

113111-7 Malzer et al. Re v . Sci. Instrum. 89 , 113111 (2018)
and signal le vel, and this will be performed on a case-by-case
basis.
CONCL USIONS
A spectrometer for X-ray emission spectroscopy measure-
ments in the laboratory was b uilt and characterized. Sample
handling in an inert N 2 -atmosphere and optional cryogenic
cooling facilitate the analysis of reacti ve chemic al compounds.
Its high sensiti vity enables experiments with dilute samples
with acquisition times of typically 5–10 h.
In comparison with the prototype instrument, the current
yields much higher quality of the XES spectra. The spectral
resolving power is no w E/ ∆ E = 4000 instead of E/ ∆ E = 2000. In
addition, the peak tailing was reduced significantly . The second
important progress is the extended range of elements, which
can be analyzed. Spectra of sulfur compounds can be acquired,
whereas the lightest element accessible with the first setup was
calcium. In particular for dilute specimen, sample preparation
is crucial in order to optimally utilize the laboratory XES-
spectrometer’ s capabilities.
A CKNOWLEDGMENTS
S.D. ackno wledges the Max Planck Society for fund-
ing. The de velopment of the spectrometer was funded by the
European Research Council under the European Union’ s Se v-
enth Frame work Programme (No. FP/2007-2013) ERC Grant
Agreement No. 615414.
APPENDIX: FUND AMENT ALS OF HAPG OPTICS
Highly Oriented Pyrolytic Graphite (HOPG) is a kind of a
niche material with respect to X-ray optics. By discussions and
also thanks to the re view process of this paper , we were made
aw are that kno wledge on this material and on the fundamentals
of optics made from mosaic crystals is not as widespread as
for ideal crystals. T o spare the interested reader the burden to
search for that kno wledge which is scattered in the literature of
three decades, we compiled this short summary . It answers fre-
quently asked questions and pro vides references to a selection
of publications.
Proper ties of HOPG and HAPG
HOPG is a mosaic crystal, which consists of small crystal-
lites of graphite with a thickness of ca. 1 µ m. A breakthrough
for the fabrication of X-ray optics w as the dev elopment of
flexible HOPG by Antono v and Grigorie va in the be ginning
of the 90s. 18 Sheets of flexible HOPG with a thickness do wn
to ca. 10 µ m 19 can be mounted on substrates at room tem-
perature. A re view on its properties and application in X-ray
spectroscopy can be found in Ref. 20 . In 2006, again Antono v
and Grigorie va dev eloped Highly Annealed Pyrolytic Graphite
(HAPG). Optics made from sheets of HAPG can ha ve mosaic
spreads of do wn to 0.6 ◦ and can provide higher spectral resolu-
tion than HOPG. 21 – 23 The mosaic spread depends on the adhe-
si ve forces between substrate and the graphite sheets. The best
results were obtained with polished glass molds. 23 If graphite
optics with e ven lo wer mosaic spread can be manufactured,
and if there is a theoretical limit, is not kno wn.
W ith view to high resolution X-ray spectroscop y , the most
important characteristics of HAPG are: (a) Optics can be pro-
duced from HAPG sheets as thick as 20 µ m and with mosaic
spreads of 0.06 ◦ , (b) these sheets can be cold-mounted to sub-
strates of virtually arbitrary shape, (c) an increase in mosaicity
or Darwin width due to bending is not reported.
Reflecti vity measurements sho w that kinematical theory 24
provides reasonable accurac y for reflecti vity calculations of
HAPG. 25
While the v alues of mosaicity and reflectivity agree across
published experimental results, measured v alues for the Dar-
win width are sparse and considerably scatter . 23 W e use the
v alue giv en by Ice and Sparks 15 because it produced the best
match of estimates of the spectral resolution and of simula-
tions 16 in comparison with measurements. The manufactur -
ers recommend to use 3.354 Å as a v alue for the interlayer
distance. 26
v on Hamos spectrometers
T o the best of our knowledge, the first who published the
use of HOPG in v on Hamos geometry were Ice and Sparks
almost 30 years ago. 15 In particular , the impact of mosaicity
to the spectral resolution is discussed comprehensi vely . W e
lar gely follo w the concepts, notions, and equations published
in this highly recommendable paper .
Ice and Sparks calculate the spectral broadening by the
square sum of a number of contrib utions. Source size and
spatial resolution of the detector are taken directly . The same
holds for the Darwin width. The broadening due to mosaicity
is modeled with three components, denoted as flat focusing
error , penetration error , and roughness error . The flat focusing
error stands for the image error due to the crystallites in the
surface plane. The penetration error stands for the smearing
due to penetration of X-rays. In our calculations, the surface
roughness is neglected.
W e checked these equations by comparing to more elabo-
rate simulations 16 and experiments. F or high resolution optics,
where the contrib ution of the Darwin broadening dominates,
they pro vide good estimates for the spectral resolution.
The crystal related image errors decrease with decreas-
ing mosaicity . W ith the exception of the flat focusing error
and the Darwin broadening, all contrib utions show a constant
broadening in space. Enlar ging the distance between the com-
ponents, i.e., increasing the curv ature radius, reduces their size
in relation to the wa velength dispersion. In conclusion, a thin
crystal (20 µ m–40 µ m), a lo w mosaicity ( < 0.1 ◦ ), and a large
bending radius ( > 100 mm) are required for high spectral reso-
lution. The maximum achie vable resolution is determined by
the Darwin width like for ideal crystals.
For estimates of the ef fectiv e solid angle cov ered by a
v on Hamos optic, we use the results for peak reflectivity and
mosaicity gi ven in the work of Gerlach et al. 23 One may also
use the work of Zastrau et al. 25 The ef fecti ve solid angle
of the v on Hamos optic is estimated by the product of the
integral of the Cauch y profile of the mosaic spread (i.e., the
integral reflecti vity) times the sagital angle, the optic spans,
times the sine of the Bragg angle. W e note that for higher

113111-8 Malzer et al. Re v . Sci. Instrum. 89 , 113111 (2018)
ener gies, the profile may be clipped by the finite meridional
length of the crystal and the limits of integration must be set
accordingly . For the ef fecti ve solid angle of the entire ana-
lyzer , also the quantum efficienc y of the detector is taken into
account.
W e want to conclude with a fe w final and summarizing
remarks on the most important dif ferences between HAPG
based v on Hamos spectrometers and the ones based on the
more frequently used ideal crystals. Ideal crystals are a vailable
in v arious cuts and materials. For the design of spectrometer ,
frequently crystals are selected which allo w for high Bragg
angles. High Bragg angles are adv antageous with respect to
the spectral resolution, solid angle of detection, and instrument
size. HAPG can only be used in the 002 plane or in second and
higher orders. Ho wev er , due to the loss of reflecti vity already
in the second order of reflection, this option is not attracti ve for
laboratory instrumentation. The bending radius and the thick-
ness of the crystal are the two major parameters determining
spectral resolution of the optic.
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