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572 doi:10.1107/S0021889809006803 J. Appl. Cryst. (2009). 42, 572–579
Journal of
AppliedCrystallography
ISSN 0021-8898
Received 19 March 2009
Accepted 19 May 2009
# 2009 International Union of Crystallography
Printed in Singapore – all rights reserved
An efficient X-ray spectrometer based on thinmosaic crystal films and its application in variousfields of X-ray spectroscopy
Herbert Legall,a* Holger Stiel,a Matthias Schnurer,a Marcel Pagels,a Birgit
Kanngießer,b Matthias Muller,c Burkhard Beckhoff,c Inna Grigorieva,d Alexander
Antonov,d Vladimir Arkadieve and Aniouar Bjeoumikhovf
aMax-Born-Institut fur Nichtlineare Optik und Kurzzeitspektroskopie, Max-Born-Strasse 2A, 12489
Berlin, Germany, bTechnical University Berlin, Institut fur Optik und Atomare Physik, Harden-
bergstrasse 36, 10623 Berlin, Germany, cPhysikalisch-Technische Bundesanstalt, Abbestrasse 2-12,
10587 Berlin, Germany, dOptigraph GmbH, Rudower Chaussee 29/31, 12489 Berlin, Germany,eIAP – Institut fur Angewandte Photonik eV, Rudower Chaussee 29/31, 12489 Berlin, Germany,
and fIfG Institute for Scientific Instruments GmbH, Rudower Chaussee 29/31, 12489 Berlin,
Germany. Correspondence e-mail: [email protected]
X-ray optics with high energy resolution and collection efficiency are required in
X-ray spectroscopy for investigations of chemistry and coordination. This is
particularly the case if the X-ray source emits a rather weak signal into a large
solid angle. In the present work the performance of a spectrometer based on
thin mosaic crystals was investigated for different spectroscopic methods using
various X-ray sources. It was found that, even with low-power X-ray sources,
advanced high-resolution X-ray spectroscopy can be performed.
1. Introduction
The applicability of X-ray spectroscopic methods for the
investigation of chemical states and coordinations is in some
cases limited by the collection efficiency and the energy
resolution of the X-ray optics. A typical example is chemically
sensitive X-ray emission spectroscopy (Glatzel & Bergmann,
2005), which needs both high energy resolution and high
collection efficiency. In other cases the feasibility of an
experiment can be limited by the low flux supplied by the
X-ray source that is used for the experiment. Prominent
examples include femtosecond-pulse-emitting laser-based
X-ray sources, e.g. laser plasma sources (Thoss et al., 2003;
Zhavoronkov et al., 2005). While in the first case the signal is
emitted in a large solid angle, in the second case the source
used for the experiment radiates isotropically in all directions.
In both cases the solid angle of collection of the X-ray optics
should be as large as possible. Single crystals, which are
commonly used in experiments in which high energy resolu-
tion at photon energies above 1 keV is required, exhibit only
small acceptance angles and the integrated reflectivity is
therefore low. Bending these crystals (Johannson geometry)
increases the collection angle, but single crystals can be bent
only weakly, and large-area crystals are in general not avail-
able. Consequently a multi-crystal analyzer is used to ensure a
large solid angle of detection (Glatzel & Bergmann, 2005).
In the present work another strategy is described, which
makes use of the efficient collecting properties of mosaic
crystals. An ideal mosaic crystal consists of a large number of
small crystallites that are randomly tilted over a small angular
range perpendicular to the surface normal (cf. Fig. 1).
Consequently, these small crystallites can cover a large solid
angle in the scattering plane, even if the surface of the crystal
is flat. The FWHM of the angular distribution of the crystal-
lites is called the mosaic spread. For very thin pyrolytic
graphite crystals, as used in this work, a Lorentzian distribu-
tion was found (Legall, Stiel, Antonov et al., 2006). Because
the mosaic spread of these crystals is much larger than the
single-crystal rocking curve width, the integrated reflectivity
of mosaic crystals can be some orders of magnitude larger than
that for single crystals (Beckhoff et al., 1996; Legall, Stiel,
Arkadiev et al., 2006). If the intrinsic reflection width (Darwin
width for perfect single crystals) of the individual crystallites is
small, high energy resolution can be observed even with
mosaic crystals (Legall, Stiel, Antonov et al., 2006; Legall,
Stiel, Arkadiev et al., 2006). Furthermore, these crystals can be
made in a strongly bent three-dimensional geometry
(Grigorieva & Antonov, 2003), and bending effects on the
reflection properties of the small crystallites are small.
Consequently, nearly no loss in energy resolution is observed
(Legall, Stiel, Antonov et al., 2006). Finally, virtually no area
limitations exist, because the area of mounted foil crystals is
only limited by the size of the substrate.
Highly oriented pyrolytic graphite (HOPG) is a well known
representative of mosaic crystals and can be used in an energy
range from 3 keV up to several tens of kiloelectron volts
(Beckhoff et al., 1996). In addition, it was shown recently that
these crystals can be produced with low intrinsic reflection
broadening, which is a pre-condition for high spectral reso-
lution. This new type of thin pyrolytic graphite, called HAPG
(highly annealed pyrolytic graphite), developed by Optigraph
GmbH for these applications and originally described by
Legall, Stiel, Antonov et al., 2006), was used in this work.
Experiments covering various spectroscopic methods have
been performed with a variety of X-ray sources to demon-
strate the applicability of a HAPG spectrometer if a high
energy resolution and a high collection efficiency are required.
2. Diffraction properties of the HAPG mosaic crystalfilms
The mosaic spread influences the diffraction properties of a
mosaic crystal in several ways: it determines the integrated
reflectivity (acceptance angle for monochromatic radiation)
and enables broad band reflection for a given angle of inci-
dence. Furthermore, it affects the penetration depth (as a
result of extinction effects), which contributes to energy
resolution (Fig. 1). In addition, in a symmetric geometry,
mosaic focusing (parafocusing) takes place in the scattering
plane. This enhances the intensity in the image plane as well as
the spectral resolution.
The energy resolution is determined by various factors,
which give rise to a smearing in the image plane. Some of these
factors are independent of the chosen distances, such as, for
example, the intrinsic reflection broadening of the crystallites
and the so called ‘flat focusing error’ (Ice & Sparks, 1990). The
latter is a consequence of crystallites not lying on the Rowland
circle in a flat crystal geometry. The contributions of other
factors change with distance. If a constant spatial smearing �s
is induced in the image plane – for example, as a result of
penetration effects into the depth of the mosaic crystal or
simply because of the imaged source size – the contribution of
these factors to the energy resolution can usually be neglected
at larger distances. The reason for this is an increasing spectral
dispersion in the image plane with distance. In Fig. 2 the
influence of the different contributions to an energetic
smearing as a function of distance is displayed. The spectral
resolution of the crystal can be obtained by convolution of
these contributions. The formulae used for these calculations
are given, for example, by Ice & Sparks (1990).
As discussed previously, the mosaic spread affects the beam
penetration. The penetration depth is lower for small mosaic
spread and can limit beam penetration (the screening effect)
more than absorption. In addition, low mosaic spread strongly
reduces the flat focusing error. To summarize, if the source size
and the mosaic spread are small the spectral resolution of a
mosaic crystal can be improved by increasing the distances F
between the source and the crystal and the detector plane,
respectively. The spectral resolution in this case is, similar to
the case for single crystals, limited by the intrinsic reflection
broadening of the small crystallites. The intrinsic reflection
broadening is a consequence of particle size and strain and
gives therefore an upper limit for the spectral resolution. This
upper limit for the energy resolution of the mosaic crystal is
given by the dispersion relation, which can be written as
E=�E ¼ tan �=��intr; ð1Þ
where � is the Bragg angle and ��intr is the averaged intrinsic
width of the Bragg reflection.
The integrated reflectivity is proportional to the number of
crystallites participating in the reflection process. If the
penetration depth is large (e.g. because of large mosaic
spread) or multiple reflections occur in the crystal, losses due
to scattering and absorption must be considered. Both the
atomic scattering and the absorption depend on photon
energy. Consequently, the integrated reflectivity depends on
the photon energy too, as well as the energy resolution
(Beckhoff et al., 1996; Freund et al., 1996; Alianelli et al., 2001).
It is very important to note that, in contrast to the energy
resolution, the effective integrated reflectivity is independent
of the distance between crystal, source and detector (beyond a
certain minimum distance from which the incident angular
emission characteristic can be considered to be constant). If
the crystal size is adapted to the distance, or in other words, if
the same solid angle is collected at all distances, the same
intensity in the image plane is observed. The latter means that
the flux that can be collected, and hence the intensity in the
research papers
J. Appl. Cryst. (2009). 42, 572–579 Herbert Legall et al. � An efficient X-ray spectrometer 573
Figure 2Different contributions to a smearing in the image plane and itsdependence on distance calculated for 8 keV. The formulae forcalculation are given, for example, by Ice & Sparks (1990).
Figure 1Illustration of mosaic focusing which takes place if the distances Fbetween the crystal and the source and the image plane are equal. Alsoshown is the Rowland circle and the focusing errors arising fromreflection from deeper lying crystallites in the film.
image plane, is an intrinsic property of the crystal and does not
change if the energy resolution is maximized by optimizing the
spectrometer geometry. A trade off between energy resolution
and collected flux only arises if the thickness and/or the mosaic
spread of the crystal is varied.
3. Spectrometer setup
The performance of a wavelength dispersive spectrometer
depends on the spectrometer geometry, the crystal and the
detector. Bending the crystal in a cylindrical geometry and
placing it between source and detector in the image plane
results in a so-called ‘von Hamos spectrometer’ setup (von
Hamos, 1932). In this geometry the incident radiation can be
collected in a large solid angle (cf. Fig. 3), while in the
dispersion plane the crystal behaves like a flat crystal (Rm =
1).
If the radius (Rs = F sin �) of the crystal cylinder is chosen
properly the radiation collected from the source will be
focused in the image plane geometrically. Owing to the mosaic
spread, the image of the source shows an additional spread
which takes place in the direction out of the dispersion plane
(cf. Fig. 3). For lower mosaic spread this smearing out of the
dispersion plane is lower and better focusing can be obtained
(Sanchez del Rio et al., 1998).
The spectrometer in this work was designed to function in
polychromatic mode and no entrance and/or exit slits were
used. The entrance slit of the spectrometer is the source size
itself. In this mode the whole spectrum can be collected
simultaneously, if an appropriate detector (e.g. a CCD
camera) is used. The advantage of this mode is that it enables
single-shot spectroscopy if a pulsed X-ray source is used. The
disadvantage of a slitless spectrometer is that its energy
resolution is critically influenced by the source size. By placing
an area detector on the cylinder axis (Shevelko et al., 2002) the
energy range for which geometrical focusing in the detector
plane occurs can be, in principle, extended without limitations
(cf. Fig. 4). However, for high energy resolution at large
distances with the latter configuration, a detector with a large
detection area is required. Additionally, if a cooled area CCD
is used the whole setup must be placed in a vacuum, because it
is difficult to realize low incident angles if a window for
vacuum sealing is used. Therefore, the detector plane was
placed perpendicular to the central ray of the spectrum
reflected by the crystal (cf. Fig. 4), as was also done by Ice &
Sparks (1990). In this geometry for a certain energy E0
parafocusing can take place, while other energies are focused
in front of or behind the detector plane. Consequently, in this
geometry the energy resolution drops for energies not equal to
E0. The extent of this loss in energy resolution depends
strongly on the mosaic spread. For the experiments in this
work we found a loss of less than 10% in energy resolution
over the collected energy range of interest. The energy
dispersion in the image plane can be calculated in this
geometry by the following equation:
�xð�Þ ¼ 2F tanð�0 � �Þ: ð2Þ
Here �xð�Þ is the distance of the image of a variable energy E
with respect to that of E0 in the detector plane. The angles �0
and � are the Bragg angles for the two different energies.
Equation (2) can be use to calculated the energy range that
can be collected simultaneously by using an area detector at a
certain distance F. For example, at a distance of 680 mm at
8.3 keV a spectral range of 1 keV can be observed with a
detector length of 25.4 mm.
The dispersive elements in the HAPG spectrometer were
100 mm-thick HAPG crystals. The crystals were bonded by
Optigraph GmbH to cylindrically polished glass substrates
using a glue-free process. This process avoids distortions that
typically arise from non-uniform glue layers and ensures high
surface quality of the crystal optic. The intrinsic width of
reflection of these crystals at 8 keV was estimated to be 27
arcseconds for the 002 reflection from rocking curve
measurements performed with a 15 mm-thick crystal (Legall,
Stiel, Antonov et al., 2006). This very thin crystal was taken
from the same production batch as those used in the present
work. The mosaic spread of this very thin HAPG crystal was
measured to be 0.04�. For thicker HAPG crystals of 100 mm no
rocking curves have been measured before now, but according
to Grigorieva & Antonov (2003) the mosaic spread of this type
of crystal increases with thickness. The thicker HAPG crystal
was chosen for the experiments in this work because the
integrated reflectivity increases with both the thickness and
the mosaic spread (see above). Furthermore, a larger mosaic
spread increases the acceptance angle of the crystal; this is
favorable if the spectrometer is operated in a polychromatic
mode, because a modulation in reflectivity caused by mosaic
research papers
574 Herbert Legall et al. � An efficient X-ray spectrometer J. Appl. Cryst. (2009). 42, 572–579
Figure 3Sketch of the von Hamos spectrometer.
Figure 4Modified von Hamos geometry.
spread restricts the energy range that can be collected simul-
taneously with a given crystal size.
The integrated reflectivity can be estimated from the
rocking curve measurements, which reveal the reflectivity over
the incidence angle of X-ray radiation. For the 15 mm HAPG
crystal the rocking curve gave an integrated reflectivity of
0.7 mrad at 8 keV in the first reflection order, which is nearly
ten times larger than that reported for Ge(111) crystals (Town,
1997). From measured Cu K� emission spectra a decrease in
intensity in the image plane of a factor of five was found if
working with the second-order reflection of the same crystal.
Comparing the measured Cu K� emission spectra of the 15
and 100 mm crystals gives six to seven times more intensity in
the image plane for the thicker crystal.
To collect a broad spectral range, a 16 bit deep depletion
CCD camera (Roper Scientific model PI-LCX 1300) with a
quantum efficiency of about 50% at 8 keV was placed in the
image plane. The CCD was calibrated with an Fe55 radioactive
source. In this setup, the pixel size of 24 mm determines the
accuracy of the absolute calibration of the energy axis in the
detector plane (see below). A thin (250 mm) Be window in
front of the deep depletion CCD was used for vacuum sealing
of the camera, so that a deep cooling (down to 223 K) of the
CCD was possible.
4. Experiments
The spectrometer concept was tested in various experiments
using three different types of X-ray sources. The experimental
conditions were chosen such that the flux emitted by the X-ray
source is at the limit of the detection efficiency of today’s
commonly used spectrometer setups. The X-ray sources were
a low-power micro-focus X-ray tube with an Ag anode
(iMOXS MFR; IfG GmbH) for EXAFS measurements, the
mySpot beamline at BESSY II for the X-ray fluorescence
(XRF) experiment and an ultrashort laser plasma source
emitting femtosecond X-ray pulses at the Max-Born-Institut.
4.1. X-ray absorption measurements
The micro-focus tube used in the EXAFS experiments
delivers a Bremsstrahlung continuum with a flux of about
109 photons s�1 sr�1 in 0.1%BW (bandwidth) at 8.3 keV when
the tube is operated with a voltage of 40 kV and an anode
current of 800 mA (cf. x4.3). The source size of the X-ray tube
was 50 mm. Measurements were performed for the 002 and 004
reflections. The energy resolution of the HAPG crystal used
for these measurements was determined from Cu K� spectra
at 8 keV to be E/�E = 1800 for the 002 reflection at a distance
of 680 mm (which represents an upper limit for the 100 mm
crystal at this reflection order) and E/�E = 2100 at a distance
of 360 mm for the 004 reflection. At these distances the
spectral position of the Ni K-edge on the CCD is close to the
energy that is best focused by a cylindrical HAPG with a
radius R = 150 mm (cf. Fig. 5). The length of the cylindrical
crystal was 3 cm and the height was 5 cm. The EXAFS
measurements were performed in transmission with a 4 mm Ni
foil as sample placed between the source and the crystal.
Images were collected with and without a sample. The images
of the Bremsstrahlung continuum after transmission through
the Ni foil are shown in Fig. 5 for both reflection orders.
From the measured images, the Ni K-edge EXAFS spec-
trum can be calculated if the transmitted signal behind the
sample is divided by the unfiltered Bremsstrahlung continuum.
As a result of this division, a modulation of the spectral
intensities by the mosaic crystal can be ignored in the
evaluation procedure. If the geometrical acceptance angle of
the mosaic crystal is too small to reach a low noise-to-signal
ratio over the whole spectrum the energy range must be
scanned. In the presented EXAFS experiment two scans were
sufficient. From the spectra, the noise-to-signal ratio after
20 min accumulation time was determined to be 1% for the
002 reflection and 5% for the 004 reflection. After subtracting
the atomic background absorption the EXAFS oscillation can
be separated from the measured spectrum. Calibration of the
energy axis was achieved by using an Ni foil reference spec-
trum (Farrel Lytle database; http://ixs.iit.edu/data/Farrel_
Lytle_data/PROCESSED/n/nifoil.k45). The result of this
procedure is shown in Fig. 6. For the sake of comparison the
reference spectrum is displayed as well.
As can be seen from Fig. 6, with the HAPG spectrometer
setup, EXAFS spectroscopy can be performed with laboratory
low-power X-ray sources. A comparison of the HAPG spec-
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J. Appl. Cryst. (2009). 42, 572–579 Herbert Legall et al. � An efficient X-ray spectrometer 575
Figure 5EXAFS images and cross sections. The images show the Bremsstrahlungcontinuum after transmission through an Ni foil of 4 mm thickness. Theacquisition time for the images was 20 min.
trometer with the spectrometer setup presented by Benesch et
al. (2004) shows the advantage of using thin mosaic crystals as
was done in this work. For EXAFS measurements, Benesch et
al. (2004) used a single Si(111) crystal combined with a
capillary lens for focusing X-rays emitted by a micro-focus
X-ray tube (TruFocus TFX-8050, 15 W) on a liquid sample.
Owing to the use of the capillary lens, the flux collected by the
dispersive element after transmission through the sample is
enhanced. On the other hand, the source size given by the
focal spot of about 100 mm on the sample decreases the energy
resolution significantly. The acquisition time for an EXAFS
spectrum reported by Benesch et al. (2004) was 20 h instead of
20 min as reported in this work, and the energy resolution was
E/�E = 1400 at 7–8 keV. Considering the lower power (15 W)
of the X-ray tube used by Benesch et al. (2004) in comparison
with the X-ray tube used in this work (30 W), there is still a
factor of 30 gain in spectrometer efficiency.
4.2. X-ray emission spectroscopy (XES)
Chemically sensitive X-ray 2p–1s (K�) or 3p–1s (K�) K
fluorescence spectroscopy can be performed if the interaction
between the 2p and 3p wavefunctions and the wavefunction of
the binding orbital is large enough to show ligand-dependent
chemical shifts (Glatzel & Bergmann, 2005). This is the case,
for example, in 3d transition metal complexes. Appropriate
targets of these investigations are then, for example, the K�satellite lines, which mainly arise from transitions out of
orbitals higher than the 3p shells. Such investigations are today
usually performed at synchrotron undulator beamlines
(Glatzel & Bergmann, 2005; Reinhardt et al., 2009), because
the signal flux emitted from these satellite transitions is quite
low. In this section, measurements performed at a synchrotron
wavelength-shifter beamline in combination with the HAPG
spectrometer will be presented.
The samples investigated in X-ray emission measurements
were a Ti foil and a Ti oxide compound. The radiation of the
mySpot beamline at BESSY II was focused with a poly-
capillary lens (throughput 40%) on the sample, resulting in a
focal spot size of 30 mm with a flux of about 1010 photons s�1
at 8.5 keV. This is a much lower flux than the 1012–
1013 photons s�1 delivered by undulator beamlines. However,
this higher flux is not useful in an experiment in which the
damage threshold of the sample is exceeded. In this case it can
be helpful to optimize the collecting X-ray optics instead of
increasing the incident flux on the sample to resolve these
weak XRF signals. Taking the flux of the mySpot beamline at
BESSY II into account for the K�5 emission, a signal inten-
sity of about 6 � 103 photons s�1 sr�1 was calculated and
confirmed in the experiment. The XES measurements in this
work were performed with a 100 mm bent HAPG crystal
(radius 100 mm, length 6 cm and height 5 cm) placed at a
distance of F = 250 mm between the sample and the CCD.
In Fig. 7 the measured Ti and TiO2 K� XRF spectra are
displayed. Different contributions to the K� emission spec-
trum could be identified. The spectra were fitted with
Lorentzian line profiles, except for the radiative Auger KMM
contribution, which was estimated to have a Gaussian line
shape because of the multiplicity of lines. For K�1;3 in the case
of the Ti foil two Lorentzian profiles were needed, whereas in
the case of TiO2 three Lorentzian profiles were used to fit the
spectra. The latter is a consequence of the strong asymmetry
of K�1;3, which can also be explained by a modulation of the
intensity due to the reflection curve of the mosaic crystal. A
research papers
576 Herbert Legall et al. � An efficient X-ray spectrometer J. Appl. Cryst. (2009). 42, 572–579
Figure 6Measured EXAFS oscillations after subtraction of the atomic back-ground. Also shown is the Ni foil reference spectrum (Farrel Lytledatabase) used for calibration of the energy axis.
Figure 7Measured K� fluorescence of a Ti foil and a TiO2 pellet using the HAPGspectrometer. The acquisition time was 7 h for the Ti spectrum and 3 h forthe TiO2 spectrum.
comparison of the spectra measured in this work with spectra
of other authors shows that the line intensity in this work is
tailed off by the reflection curve of the crystal, which was
aligned to maximum reflectivity for the energy of the K�5 line.
Therefore the real intensity of the satellites can only be
recalculated from the spectrum once the reflection curve is
known. However, the inaccuracy of this procedure is relatively
large considering the weakness of the fluorescence lines. An
experimental improvement is necessary at this point. The best
way to eliminate such modulation effects is a calibration of the
spectrometer, either by employing radiation scattered elasti-
cally at a thin sample in the XRF beam geometry or by direct
illumination of the HAPG crystal with monochromatic
radiation. Both approaches require the tunability of
synchrotron radiation and knowledge of the radiant power of
the radiation. The incident radiation power can be determined
by photodiodes calibrated absolutely (Beckhoff, 2008;
Beckhoff et al., 2006).
However, from the pixel size and the distances between the
K� and K� lines the accuracy of the measurement can be
calculated to be �0.2 eV with 0.4 eV per pixel. As mentioned
above the accuracy of this experiment can be enhanced
without problems by increasing the distances. No loss in
intensity on the CCD is expected if the crystal size is enhanced
to collect the same solid angle. In the same way the energy
resolution can be significantly improved. In Table 1 the
energies of the second-order contributions in the K�1;3 spectra
are tabulated and compared with the results of other authors.
4.3. Plasma emission spectroscopy
Ultrafast laser-based plasma sources deliver X-ray pulses
with durations of some hundreds of femtoseconds and enable
time-resolved investigations with X-ray techniques such as
diffraction (Bargheer et al., 2004) and absorption spectroscopy
(Lee et al., 2005). Up to now only a few papers have been
published dealing with time-resolved EXAFS experiments in
the picosecond (Bressler et al., 2002; Saes et al., 2003, 2004;
Bressler & Cherugi, 2004; Audebert et al., 2005; Chen et al.,
2002) or sub-picosecond range (Lee et al., 2004; Benesch et al.,
2004). Time-resolved spectroscopy using femtosecond laser
plasma sources is still a challenge because the photon flux
delivered by laser plasma sources and other available ultra-
short X-ray sources is much lower than that obtained using
synchrotron radiation, albeit providing a lower pulse duration
limit of about 20 ps. Typical acquisition times reported for an
EXAFS spectrum with femtosecond laser plasma sources are
currently 10 h (Benesch et al., 2004). However, the ultrashort
pulse duration warrants further effort to reduce the acquisi-
tion time by enhancing the available photon flux in the
experiment. This can be achieved either by laser system
upgrades (repetition rate and in some cases laser pulse
energy) or by optimizing the spectrometer concept. The latter
we prove in the present work by applying the spectrometer for
plasma emission spectroscopy.
The laser plasma source used for the experiments consists of
a tape target system in a vacuum chamber and an off-axis
parabolic mirror as focusing element. Thin metal tapes (Cu, W,
thickness 50 mm) were illuminated using the high-field MBI
Ti:Sa laser facility (815 nm center wavelength, 40 fs pulse
duration, > 1 J pulse energy after compression, 10 Hz repe-
tition rate and contrast ratio down to 10�7 and 10�8 at peak
intensity). In the experiment, a maximum energy of 450 mJ
was applied. It was found that 35% of the energy incident on
the parabolic mirror was focused into a spot of 10 mm
diameter, giving an intensity of 3 � 1018 W cm�2 at the target.
The focused laser beam hits the surface of the target at an
incident angle of 45�. The X-ray emission was measured in
reflection at 90� to the incident laser beam. In Fig. 8 a single-
shot Cu K� emission spectrum of the laser plasma is shown;
this was collected with the HAPG spectrometer after exposing
the Cu target to a 430 mJ laser pulse with a 40 fs pulse dura-
tion. The spectrum corresponds to an averaged X-ray photon
flux of 2 � 1010 photons s�1 sr�1 in Cu K� at a laser intensity
of 3 � 1018 W cm�2. In Fig. 9 the W L� flux and the emitted
Bremsstrahlung in 0.1%BW evaluated from measured spectra
with a W target is depicted as a function of the single-shot
laser pulse energy.
The conversion efficiency for the laser-based generation of
ultrashort X-ray Bremsstrahlung scales with ðI�2Þ1=2, where I is
the laser intensity and � the wavelength (Yu et al., 1999).
Therefore, the conversion efficiency is significantly higher if
using higher laser intensities, as was done in the present work.
Comparing the 6 � 107 photon s�1 sr�1 in 0.1%BW measured
in the present work for a tungsten target at an intensity of
about 3 � 1018 W cm�2 (400 mJ pulse�1) and 10 Hz with the
2.4 � 108 photons s�1 sr�1 measured for a copper target at an
intensity of about 1016 W cm�2 (4.5 mJ pulse�1) and 2 kHz in
about 12%BW (Benesch et al., 2004) gives 30 times higher
photon numbers per second and 40 times higher conversion
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J. Appl. Cryst. (2009). 42, 572–579 Herbert Legall et al. � An efficient X-ray spectrometer 577
Table 1Energies of the second-order contributions in the K� fluorescencespectra relative to the K�1;3 contributions (line energies in eV).
A shift of the K�1;3 lines between mono-elemental Ti and the Ti compoundTiO2 could not be resolved with the present accuracy. A reported value is0.2 eV (Koster & Mendel, 1970). The uncertainties of the tabulated energypositions in this work are calculated from the pixel size and from theuncertainty in data fitting. The deviation of the energy scaling from linearity isaccording to equation (2) less than 0.05 eV for all contributions and wasneglected in the evaluation of the measured spectra.
Lines Author Ti TiO2
K�0 This work �15.7 (13) �15.6 (9)Ursic et al. (2003) �5.62 –Salem et al. (1976) �7.08 –Faessler (1955) – �16
K�00 This work 13.1 (17) 16 (5)Koster & Mendel (1970) – 15.2
K�5 This work 30.5 (4) 31.5 (4)Koster & Mendel (1970) 30.9 31.4Ursic et al. (2003) 29.96 –Ursic et al. (2003) – 30.4
K� L1 This work 52.2 (22) 49 (29)Ursic et al. (2003) 50.4 –Raju et al. (2007) 53.5 –
efficiency for the 10 Hz high-field laser plasma source. Besides
the higher photon flux delivered by the high-field laser system,
the lower repetition rate is desirable in time-resolved pump-
probe spectroscopy because the sample is heated much less by
an exciting UV/VIS pulse after ten photocycles per second
than after the 1000 cycles that occur in the kiloherz mode. On
the other hand, the generation of X-rays using high intensity
increases drastically the amount of background X-ray radia-
tion because of the interaction of the highly energetic elec-
trons with the surrounding medium. This means a careful
shielding of the CCD is a pre-condition for obtaining a high
signal-to-noise ratio.
In Table 2 the photons collected on the CCD using the laser
plasma source are related to the photons collected using the
micro-focus X-ray tube that was used for the EXAFS
experiment in x4.1. The setup was identical in both measure-
ments. For the measurements, the target in the laser plasma
chamber was simply replaced by the tube. A comparison of the
flux collected for both sources allows an estimation of the
acquisition time, which is necessary for measuring an EXAFS
spectrum with the HAPG spectrometer using the laser plasma
source instead of the micro-focus X-ray tube.
As can be seen from Table 2, the laser plasma source can be
expected to require a 10� (800 mA/600 mA) = 13 times longer
acquisition time to obtain an EXAFS spectrum that has the
same accuracy (under low background scattering conditions)
as was obtained with the micro-focus X-ray tube in x4.1. This
acquisition time can be further reduced by a factor of 2–3
using a high-Z target material such as tungsten (cf. Fig. 9),
because the conversion efficiency scales with the atomic
number Z. The latter would result in an acquisition time of
about 2 h for the EXAFS spectrum of an Ni foil measured
with a femtosecond X-ray source. This is a significant reduc-
tion of acquisition time compared with the 10 h reported by
Benesch et al. (2004). Furthermore, comparison of the counts
on the CCD also indicates that a significant enhancement in
accuracy can be expected by using the HAPG spectrometer.
Comparing the 50 photons s�1 in 1 keV (or 0.4 photons s�1 in
0.1%BW) on the CCD reported by Benesch et al. (2004) with
the 2.3 � 104 photons s�1 in 0.1%BW at the maximum of the
reflected Bremsstrahlung continuum for the 002 reflection
using the HAPG spectrometer (x4.1), the number of photons
on the CCD can be expected to increase by a factor of (2.3 �
104 photons s�1)/(0.4 photons s�1)/13 = 4.4 � 103 with the
HAPG spectrometer in combination with the high-field laser
plasma source using Cu as target material.
5. Conclusions
A new spectrometer setup based on HAPG mosaic crystals
was described and applied in various fields of X-ray spectro-
scopy. It was shown that, as a result of the high integrated
reflectivity of the mosaic crystals used, the recording time for a
high-resolution EXAFS spectrum measured with a micro-
focus X-ray tube can be significantly reduced and the noise-to-
signal ratios improved in comparison with a single-crystal
spectrometer reported in the literature. Furthermore, it was
demonstrated that high-resolution X-ray emission spectro-
scopy of weak fluorescence lines can be performed using the
HAPG spectrometer even if the flux provided by the exciting
X-ray beam is in the range of that delivered by table top
sources, such as, for example, X-ray tubes. Towards time-
research papers
578 Herbert Legall et al. � An efficient X-ray spectrometer J. Appl. Cryst. (2009). 42, 572–579
Figure 9X-ray flux emitted by the femtosecond laser plasma source as a functionof the laser pulse energy. (Nph denotes the number of photons.)
Table 2Flux measurements.
Tabulated are the collected photons s�1 in 0.1%BW at 8.3 keV on the CCD tocompare the emitted flux of the micro-focus X-ray tube with a Cu anode andthe laser plasma source with a Cu target. Radiation was collected (bent HOPGcrystal, radius 50 mm, 150 mm thickness, 360 mm distance) over an angle of � =4 � 10�3 sr. The anode current of the tube with Cu as anode material was600 mA and the voltage was 40 kV, resulting in a nearly identical amount ofBremsstrahlung as obtained at the Ag anode at the same voltage and sameanode current. The laser intensity was 3� 1018 W cm�2 and the pulse durationwas 40 fs at 800 nm.
Photons s�1 in 002 reflection 004 reflection
Cu K� (LPP) 3 � 105 6 � 104
Cu K� (tube) 2 � 106 4 � 105
Continuum 0.1%BW (LPP) 1 � 103 2 � 102
Continuum 0.1%BW (tube) 1 � 104 2 � 103
Figure 8Single-shot Cu K� spectrum. The exposure time was 40 fs.
resolved X-ray spectroscopy with femtosecond time resolu-
tion, the spectrometer was applied for the characterization of
an ultrafast laser plasma X-ray source. It was shown that an
application of the HAPG spectrometer in the field of time-
resolved high-resolution femtosecond EXAFS spectroscopy
with ultrafast laser plasma sources is promising. If the crystal
size is adapted to the distance and the same solid angle is
collected by the crystal, higher energy resolution can be
realized in all application experiments by increasing the
distances between the source/sample, the crystal and the
detector without changing the spectrometer efficiency. Addi-
tionally, by increasing the crystal size the spectrometer effi-
ciency can be further enhanced. An improvement of the
HAPG spectrometer concept may be possible by using a
collecting optic to focus the radiation emitted by the source on
the sample. The applicability of different types of optics were
investigated by Bargheer et al. (2005) with respect to focusing
properties, the collecting efficiency and, something that is very
important for time-resolved studies, the conservation of the
femtosecond time structure of the X-ray pulses after reflection
by the optics. In summary, there is still great potential for
improvement of the presented HAPG spectrometer, which
may enable new developments in X-ray spectroscopy in the
near future.
This work was supported by the German national program
of supporting development, innovation and technology
(ProFIT Programm zur Forderung von Forschung, Innova-
tionen und Technologien, Land Berlin) and by EFRE Euro-
paischer Fonds fur regionale Entwicklung (grant No.
10126367).
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