X-ray fluorescence spectrometer

Spectrochimica Acta Part B 67 (2012) 57–63

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Novel parallel vacuum ultra-violet/X-ray fluorescence spectrometer A. Erko ⁎, A. Firsov, F. Senf Institute for Nanometre Optics and Technology, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Albert Einstein Str. 15, Berlin, Germany

a r t i c l e

i n f o

Article history: Received 25 September 2011 Accepted 2 January 2012 Available online 10 January 2012 Keywords: Femtosecond spectroscopy Reflection zone plates Parallel diffraction spectrometer

a b s t r a c t Novel instrumentation developments in X-ray spectroscopy for parallel spectral measurements with soft X-rays are described. The significant performance improvements are achieved utilising Fresnel diffraction from structures built onto the surface of a total external reflection mirror. An array of reflection zone plates was tested as a wavelength-dispersive fluorescence spectrometer for soft X-rays in the energy range of 100–550 eV. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Time-resolved spectroscopic experiments with synchrotron radiation form a growing field of research. Pump-probe experiments employing the regular time structure of the storage ring are of specific interest [1], while using special slicing techniques the sub-picosecond time domain has also been addressed. X-ray time-resolved experiments allow direct analysis of dynamic processes or transient species to understand the mechanisms and kinetics of chemical reactions or physical phenomena. They have taken advantage of the large developments in synchrotron radiation facilities, exploiting the high brilliance and pulsed nature of the radiation emitted from the storage rings and laser plasma sources. The recent developments in free electron laser sources open new possibilities for the time-resolved X-ray experiments utilising pulses as short as 100 fs, with ~10 12 photons per pulse [2], thus allowing femtosecond fluorescence and scattering spectral measurements in a single pulse. To record spectra in parallel—one pulse, one spectrum—very efficient optics are necessary. Such optics must focus X-rays onto the detector, provide reasonable energy dispersion and have the highest possible X-ray transmission. The solution is to use just a single optical element—a reflection Fresnel zone plate (RZP). It has been recently demonstrated at BESSY II [3] that an RZP built onto the surface of a total external reflection mirror has remarkable properties. By utilising these properties a successful application of the device, as a focusing spectrometer/ monochromator for ultrafast time-resolved experiments, has been found. When placed in front of a sample it allowed simultaneous focusing and dispersion of X-rays on the sample and transmitted ~ 20 times more flux than a classical grating monochromator on

⁎ Corresponding author. E-mail address: [email protected] (A. Erko). 0584-8547/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2012.01.001

a beamline with the same energy and time resolution. The RZP spectrometer/monochromators have been in user operation since October 2008. Since then, further applications of RZP, exploring the same principle, were established as monochromators for high harmonic generation source and THz/infrared beamline at BESSY II. The RZPs are also successfully used as nano-focusing optics for hard X-rays, beamline collimating optics [4], etc. Here, the direct synchrotron beam or fluorescent radiation from a sample is focused horizontally and vertically by the elliptical zones of an RZP. Because of the grazing incidence geometry of the device (see Section 2) the effective focal spot size, defined by the outermost zone width, is considerably smaller. A spot size of several nanometres can be obtained at the RZP focus. A record spatial resolution, down to 15 nm at photon energy of 10 keV, focused using a linear reflection zone plate with the outermost zone width ~0.71 μm, was recently published [5]. In principle, it is possible to obtain a resolution of several nanometres using a so-called “modified zone plate” [6], which exploits the first and higher orders of diffraction simultaneously. This result was theoretically predicted several years earlier [7]. In this paper a new application of RZP as a parallel femtosecond spectrometer, placed between the sample and detector, for scattered radiation and fluorescence spectroscopy is suggested. The principles of the design and the first tests of the spectrometer are discussed. 2. Reflection zone plate principles The principles of 2-D diffraction focusing optics can be described using a simple schematic of a reflection zone plate shown in Fig. 1. Here, an interference pattern is produced by the interaction of two spherical waves radiated from points A1 and A2. A cross-section of this interference pattern with a plane surface represents a reflection Fresnel zone plate, which is a 2-D object with an elliptical shape. The path difference between different Fresnel zones, corresponding to phase shift of π, is λ/2.

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A1

Array detector

A2 R1

R2

λ1

λ4

λ3

λ2

λ5

Reflection zone plates

Fluorescence microsource

Fig. 1. Zone structure of a reflection zone plate [8]. The selection rectangle points to the off-axis area of the RZP which is used in spectrometer. The optical axis lies along the x direction.

Fig. 2. The principle of parallel fluorescence spectrometer.

The design parameters R1 and R2 correspond to the distances between a source at A1 and the centre of RZP and between the RZP centre and focused image at A2, respectively. The dispersion properties of the RZP are defined by the off-axis part of the device (see Fig. 1). Different energies are focused on the same optical axis, but at different focal lengths producing a “rainbow” effect in the focal plane of the RZP. The effect can be used for spectral analysis of the fluorescent radiation of a sample. It is important that zeroth diffraction order is rejected, providing the maximum energy and spatial resolution. The number of zones along the beam direction defines the spectral resolution of the spectrometer while the outermost zone width of the RZP defines the size of the focal spot. From the grating formula dðcosα−cosβÞ ¼ nλ;

ð1Þ

where α and β are the grazing incidence angle and the reflection angle, respectively, d is the grating (zone) period and n is the diffraction order, the minimum meridional period (along the beam direction) of RZP is effectively increased by a factor of sinα and can be considerably larger than sagittal period (transverse to the beam) with the same spatial resolution. 3. Parallel fluorescence spectrometer An optical device, based on an array of RZPs placed in parallel on a single surface and which employs a 2-D dispersion principle, forms a novel type of a parallel diffraction spectrometer (PDiS). An application of the device extends the usable energy range beyond 1000 eV while providing the same energy resolution as a grating spectrometer.

detector. Therefore, a parallel sampling is possible from a singleshot X-ray fluorescence pulse. The device can be used when a limited, well selected number of fluorescence lines have to be analysed. The energy resolution is defined by the pixel size and the energy dispersion in the vertical direction. 3.2. Design of diffraction element The PDiS working range of 100–550 eV was chosen because of the lack of high-resolution parallel X-ray detectors in this range. The large energy span makes the fabrication of the diffraction element for the spectrometer, with high energy resolution, rather complicated. Currently only diffraction gratings, in combination with refocusing mirrors, have been used for scanning spectral analysis with the required spectral resolution. Fortunately, there are only a few fluorescence lines in the range. Their energies and the corresponding chemical elements are listed in Table 1. Only K and L characteristic fluorescence energies of the elements are shown because other lines have very low intensities and are difficult to register using this optical arrangement. Top view schematics of a part of the PDiS are shown in Fig. 3. Each RZP in Fig. 3 is designed for spectral dispersion around a particular characteristic fluorescence energy line. Together, the nine elements cover the full range of 100–550 eV with average resolution of ~1%. The acceptance of each RZP is 1.2 × 10 − 4 sr which gives the total fluorescence flux of the source of ~10 − 5 photons radiated into 4π. For simultaneous spectra registration the spectrometer optics should not temporally “blur” short X-ray pulses. To meet this requirement, the number of periods in the diffraction structure along the beam direction should not exceed the value

3.1. Optical principle The optical principle of PDiS is shown in Fig. 2. An array of RZPs focuses fluorescent radiation from a micrometre source onto an array detector (an array of photodiodes or a CCD). Each RZP, which may be fabricated individually, is optimised for a different fluorescence line and produces wavelength dispersion in the plane of diffraction (perpendicular to the plane of Fig. 2). A focal spot size is measured in the plane of diffraction and depends on the source size and the geometrical parameters of the optical arrangement. The device, employing an appropriate number of individual reflection zone plates, can be used in the energy range 100–550 eV, where only a few characteristic lines are generated and the possible energy shift can be successfully measured using dispersion in the vertical, diffraction, plane. In this case an array detector is necessary to record the spectra. Each RZP produces a monochromatic focal spot on the

N max ¼

Δt pulse ; Δt λ

ð2Þ

where Δtpulse is the X-ray pulse duration and Δtλ is the time delay for a path of one wavelength defined as λ/c, where c is the speed of light. For titanium L edge radiation (~450 eV) the time delay, Δtλ, is about 9.2 × 10 –18 s. Therefore, in an RZP with length of 40 mm and

Table 1 Characteristic fluorescence line energies for the reflection zone plates. Element

Be

Characteristic line (eV)

108.5 183.3 277 341

B

C

Ca (L) N

Sc (L)

392.4 395.4

Ti (L) V (L)

O

452.2 511.3 524.9

A. Erko et al. / Spectrochimica Acta Part B 67 (2012) 57–63

a)

a)

from the source 2

59

108 eV +- 0.5 eV

108 eV +- 1 eV

to the CCD

40

108.5 eV

277.0 eV

mm

341.0 eV 392.4 eV

0

395.4 eV

Vertical (µm)

183.3 eV

20 0 -20

452.2 eV

-40

511.3 eV 524.0 eV

0 -2 10

20

40

mm

b)

20

Horizontal (µm)

b) 108.5 eV

524eV +- 1 eV

524eV +- 2.5 eV

20

Vertical (µm)

1.8

mm

1.7

1.6

10 0 -10 -20

1.5

-10

0

1.4 10

20

mm Fig. 3. Top view schematics of the parallel diffraction spectrometer designed for the energy range (100–550 eV). a) A schematic showing nine RZPs placed on the same substrate. b) An expanded schematic of part of the RZP designed for the energy of 108.5 eV.

average period of 3 μm there are ~ 1300 periods and time delay is ~ 1.19 × 10 –14 s. 3.3. Raytracing simulation Simulation of the RZP spectrometer performance was done using the raytracing program RAY developed at BESSY II [9]. A special zone plate subroutine was developed, which explores the principle of two local gratings oriented in two directions: along and perpendicular to the beam. The program can calculate the periods of both gratings at each point of the optical arrangement, corresponding to the diffraction of rays propagating from a point source to the focus (see Fig. 1). In the case of rays arriving from a finite source, the diffraction direction is determined by a diffraction grating subroutine added to the RAY code. The implementation of the zone plate subroutine into the RAY code was done in cooperation with King's College London [8]. The raytracing was also used to calculate the beam size and energy resolution of the RZPs as well as the required alignment accuracy. The intensity distributions in the focal planes of the RZPs for three energies around each of the lines at 108.5 eV and 524 eV (beryllium and oxygen K characteristic fluorescence lines) are shown in Fig. 4 as examples of the calculations. With a detector pixel size of 13.5 μm the energy resolution was estimated to be ~ 1%. A typical dispersion was found to be ~25 eV/mm for the beryllium line and ~ 135 eV/mm for the oxygen line. The design parameters of each reflection zone plate were calculated using a computer program [10]. The calculation parameters were: source size, 10 μm, source to RZP distance, R1 = 40 mm, RZP to sample plane distance, R2 = 160 mm, angle of incidence, Θ = 3.5° (Fig. 1).

10

20

30

Horizontal (µm)

40

Fig. 4. Raytracing results. Intensity distributions (point diagram) in the image planes of RZPs at three energies around: a) the beryllium K characteristic fluorescence line, 108 ± 0.5 eV (left) and 108 ± ?1 eV (right) and b) the oxygen K characteristic fluorescence line, 524 ± ?1 eV (left) and 524 ± ?2.5 eV (right).

Each RZP had an offset of 70 mm with respect to the optical axis to provide an effective rejection of zero order radiation. The size of the RZPs was 40 × 0.4 mm with focal lengths of 32 mm. The groove periods along the optical axes were ~1.1–5.8 μm, while perpendicular to the axes the minimum period was ~0.25 μm. The calculations shown in Fig. 4 indicate that the optical energy resolution due to spectral dispersion is as good as 0.5%. To estimate the overall energy resolution the detector pixel size must also be taken into account. For example, with a pixel size of 13 μm (iKon-M 934 CCD camera by Andor Technologies) the energy resolution, over the whole range, is ~1%. This demonstrates that dispersion in the plane of diffraction can be used for high resolution 2-D spectral measurements. 3.4. Design and manufacture The RZP spectrometer must be optimised for the maximum reflection efficiency, minimum period and depth of profile of the zones. The total efficiency of the reflection, EFF, can be calculated by taking into account the acceptance of the optical element, Z sin(α), and the angular dependence of the reflectivity, R(λ, α), where Z is the length of the RZP along the beam, λ is the X-ray wavelength and α is the grazing angle of incidence. At the total external reflection condition EFF should be calculated for the maximum energy (minimum wavelength), at which the critical angle is the smallest. The formula used for the calculation is EFF ¼ Zsinðα ÞRAu ðλ; α Þ:

ð3Þ

Fig. 5 shows the RZP reflection efficiency, EFF, as a function of grazing incidence angle. The calculation was done for 500 eV, with gold

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A. Erko et al. / Spectrochimica Acta Part B 67 (2012) 57–63

a)

20

500 eV

15

10 1

2

3

α = 2.8°, β = 3.7°

Zone period d (µm)

Efficiency (%)

25

4

5

centre max min

5

α = 2°, β = 4.3°

α = 1.55°,β = 5.1° 0.5

Incidence angle (deg)

100

200

300

400

500

Energy (eV)

used as the reflecting material. Here, the optimal grazing angle is ~ 3.5°. The second important parameter is the depth of profile of the RZP zones. This value depends on the zone (grating) period and beam energy. Since the zone period of an RZP varies continuously (see Fig. 1), estimates for the maximum and minimum periods of the zones have to be made to calculate the optimal value for the depth. This can be done using the grating formula, Eq. (1). Fig. 6a) shows the maximum and minimum values of the periods along the beam calculated as functions of energy. The periods are calculated along the optical axis of the RZP (see Fig. 1). The periods perpendicular to the optical axis are limited by the resolution of the technological process, currently ~ 0.2 μm. The values of the periods were used to calculate the theoretical diffraction efficiencies of the RZPs using program REFLEC [11]. The aim was to find a value for the depth of profile which gave a reasonable efficiency over the energy range 100–550 eV, thus allowing spectroscopic applications. In Fig. 6b) the diffraction efficiencies of the RZPs are shown, calculated as the average value between efficiency at the minimum (1 μm) and the maximum (5 μm) periods. It can be seen in the figure that the optimal depth of profile for the energy range lies between 12.5 and 15 nm. The first reflection Fresnel zone plates for BESSY II beamlines were produced in 2004 as a result of cooperation between BESSY GmbH, the Leibniz Institute for Surface Modifications in Leipzig and the Microlithography and Consulting (ML&C) Company in Jena [12]. The production process included e-beam lithography and reactive ion etching followed by coating with a gold layer. The resist masks were made by an e-beam writer at ML&C Company. The zone plates were designed at BESSY II using software modified for Bragg–Fresnel lenses. Nine RZPs, designed for fluorescence emission energies in the range of 100–550 eV (see Table 1), were fabricated onto a single super-polished 50 mm diameter silicon substrate. The substrate, produced by Gooch & Housego (California) formerly General Optics (Moorpark), had a roughness of 0.2 nm rms and slope error better than 0.6 arcsec. The RZPs were fabricated in two variants: with 12.5 nm and 25 nm profile depths to provide maximum efficiency in 100–250 eV and 250–550 eV energy ranges.

b) Average diffrction efficiency (%)

Fig. 5. The reflection efficiency of a gold coated RZP as a function of grazing incidence angle at 500 eV.

16 14 12 10 8

8 nm 10 nm 12.5 nm 15 nm 20 nm

6 4 2 100

200

300

400

500

Energy (eV) Fig. 6. a) The periods, d, along the optical axis of the RZPs as functions of energy. b) The average diffraction efficiencies of the RZPs as functions of energy. The efficiencies were calculated for periods between 1 μm and 5 μm and for depths of profile between 8 nm and 20 nm.

the X-ray fluorescence source and detector to produce a source image in the detector plane. 4.1. Overall efficiency The overall efficiency is given by the product of the reflection and diffraction efficiencies. The diffraction efficiencies of the PDiS were measured using the X-ray reflectometer at the BESSY II Optical Test Beamline. The substrate with the nine RZPs was scanned at various energies in the direction perpendicular to the beam. An X-ray grating monochromator with energy resolution better than 1000 was used to provide a monochromatic X-ray beam. The beam size was limited by a 100 μm pinhole placed in front of the sample. Because the synchrotron beam is highly collimated it was only possible to measure the efficiencies and energy dispersion of the zones close to the centre of the optical axis. The measurements were done for RZPs with the optimal, 12.5 nm, depths of profile. Fig. 7 shows the diffraction efficiencies of the spectrometer at energies of 277, 395, 452 and 511 eV. The angle of incidence was 2° and the first order diffraction angle was 4.3°, as shown in the middle curve of Fig. 6a). Each RZP reflected the required energy at the design angle (see Fig. 5). The maximum measured diffraction efficiency was 15.5%, which correlates with the theoretical predictions. 4.2. Spectral resolution

4. Experimental measurements and tests The parallel diffraction spectrometer is designed for use with fluorescent X-rays produced by focusing a synchrotron beam, electron beam or source of X-rays on a sample. The PDiS is placed between

4.2.1. Experimental setup The performance tests of the spectral properties of the PDiS were done at undulator beamline UE52SGM, BESSY II. The beamline provided a monochromatic beam in the energy range of 100–1200 eV

A. Erko et al. / Spectrochimica Acta Part B 67 (2012) 57–63

61

Efficiency (%)

16

277 eV 395 eV 452 eV 511 eV

12

8

4

0 14.5

15.0

15.5

16.0

16.5

17.0

Position (mm) Fig. 7. Diffraction efficiencies of PDiS at different energies as functions of beam position on the substrate.

Fig. 9. The image in the focal plane of the diffraction spectrometer. The horizontal scale gives the design energies of the reflection zone plates 1–9.

with a spot size of 30 × 100 μm (H × V). The spectrometer was placed on a goniometer inside a custom made UHV chamber also containing a capillary collimator and sample holder. A photograph of the layout of the chamber housing the spectrometer is shown in Fig. 8. A silicon nitride membrane, 100 nm thick and covered with 100 nm layers of titanium and carbon, was used as a test sample. It was irradiated with a beam with a variable energy up to 460 eV above the absorption edge of the titanium Lα and Lβ lines (not resolved). At this energy it was possible to excite the carbon and nitrogen K fluorescence lines in addition to the titanium Lα,β but not the florescence line of oxygen Kα (524 eV) nor vanadium Lα (511 eV). A glass capillary collimator reduced the scattered beam on the sample. Fluorescent radiation from the sample was detected using a backilluminated, high-QE sensor optimised for direct X-ray detection in the 100–1000 eV range. The photon flux on the sample was estimated to be as high as 10 11 ph/s with a bandwidth, E/ΔΕ, greater than 1000. The centres of the RZPs were located at a distance of 40 mm from the source on the optical axis. The grazing incidence angle on the centres was 3.5° (design parameter). The off-axis parts of the RZPs, 40 mm long, were used to re-focus X-rays on the optical axis in the image (detector) plane. Different energies were simultaneously focused in different positions on the X-ray CCD camera.

scale gives the design energies of each of the RZPs; the upper spot for each RZP is due to carbon Kα radiation, the middle one is due to nitrogen Kα and the lower one to titanium Lα,β. The upper bright spot for RZP3 (277 eV) is the focused spot of the carbon Kα radiation, while for the other zone plates the carbon Kα radiation is out of focus and so the spot becomes increasingly larger and fainter the further the design energy is from the carbon Kα energy. The same effect is seen for the nitrogen Kα and titanium Lα,β lines and their respective RZPs. The acquisition time was 100 s, detector temperature was −60° C. The “pure” fluorescence lines had signal-to-noise ratios as high as 1500, which indicates the high quality of the measured spectra. The intensity distribution along the line A of Fig. 9 is shown in Fig. 10a). The nitrogen Kα line appears twice in this spectrum because of the very similar design energies of RZP5 and RZP6 (392 eV and 395 eV). The vertical separation of these two spots (labelled N1 and N2 in Fig. 9) was used to estimate the energy resolution of the spectrometer which was ~1.7 eV at 392 eV. The vertical dispersion can be also used for high resolution spectral measurements around the RZP design energies. Fig. 10b) shows the vertical scan of the intensity distributions, along line B of Fig. 9, for two spectral lines: nitrogen K and titanium Lα,β. The results are in agreement with model calculations by the raytracing program RAY and confirm the feasibility to perform spectral measurements in the range of 10% around the design energy. An average count rate in the focal plane of PDiS was ~25 counts/s. The full width at half maximum of the titanium Lα,β peak in Fig. 10b) was measured to be 7 pixels, which corresponds to 11.9 eV. In this particular case the resolution was limited by the beam size of 100 μm on the sample. The spectral measurements of the sample fluorescence at different energies of the primary synchrotron beam are shown in Fig. 11. The beam was scanned at 1 eV steps. The spectra were recorded in parallel in the energy range 100–550 eV, covering absorption edges of carbon, nitrogen and titanium. In effect, XANES spectra of these three materials have been recorded.

4.2.2. Results Fig. 9 shows the 2-D image, obtained with a CCD detector. The shadows of the nine reflection zone plates on the mirror-reflected beam can be seen in the bottom part of the figure. The horizontal

5. Conclusions

Fig. 8. An image of the UHV chamber housing the parallel diffraction spectrometer.

The first successful test results indicate that the parallel diffraction spectrometer can be potentially used in a variety of applications. The possibility of simultaneous, high resolution recordings of spectra in the energy range beyond 1000 eV is important for time-resolved measurements in fluorescence and scattered light. The main optical element of the PDiS, the reflection zone plate, combines reflection, focusing and dispersion simultaneously and provides high sensitivity, high signal-to-noise ratio spectral measurements with an energy resolution previously possible only with grating spectrometers. The sagittal angular acceptance of the individual RZPs can be as high as λ/δr,

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A. Erko et al. / Spectrochimica Acta Part B 67 (2012) 57–63

a)

A - scan N (Kα)

a) 2500

C (Kα)

Ti (Lαβ)

Ti Energy (eV)

2000

Counts

O

500

1500 1000

400

N Ca

300

C

500 200

B

0 800

1000

1200

1400 1

Pixel number

4

5

6

7

b) Ti (Lα,β)

1000

1000

N (Kα)

Zn Cu

900

Energy (eV)

800

Counts

3

RZP number

B - scan

b)

2

600

400

C (Kα)

200

Ni

800

Co 700

Fe Mn

600

Cr 0 1400

1500

1600

1700

500 1

Pixel number

2

3

4

5

6

7

RZP number

Fig. 10. Scans of the intensity distribution in the focal planes of the reflection zone plates array: a) along the horizontal line A and b) along the vertical line B of Fig. 9. One pixel corresponds to 1.7 eV.

Fig. 12. Combined energy dispersions of two parallel diffraction spectrometers, each consisting of seven RZPs, designed for: a) 100–550 eV and b) 550–1100 eV.

where λ is the wavelength and δr is the outermost zone width. In the meridional direction the angular acceptance is defined by the RZP length and grazing angle of incidence. Therefore, taking into account the technological limitation for the fabrication of the outermost zone width of 50 nm, the total acceptance is as high as 10 − 4 sr at a wavelength of 1 nm and focal length of 1000 mm. Continuous dispersion, required to analyse a large number of fluorescence lines in the energy range 100–1100 eV, can be achieved by combining the horizontal and vertical dispersions. Fig. 12 shows the

usable energy dispersion ranges of two PDiSs designed for energy ranges 100–550 eV and 550–1100 eV. Each reflection zone plate has the central design energy, corresponding to the fluorescence line of a chosen material, and the energy dispersion range, indicated by the error bars. The energy ranges of the different RZPs overlap each other and create a continuous diffraction spectrum, which represents a 2-D image of the fluorescent X-ray emission in a detector plane. The energy range is defined by the number of zone plates and energy resolution is defined by the vertical dispersion. Such an optical device, consisting of two or more PDiSs, can be used for measurements of a continuous spectral distribution covering a very wide energy range.

C (Kα)

N (Kα)

Ti (Lαβ)

Acknowledgements

4000 475

) (eV gy

450

400 300

0 0

200

400

600

800

me

425 1000

ner

2000

Bea

Counts

3000

1000

Pixel Fig. 11. Fluorescence spectra of the test sample for different incident energies.

We are very grateful to Prof. Dr N. Langhoff and Dr. A. Bjeoumikhov (IfG-Institute for Scientific Instruments GmbH), Dr. Y. Höhn and Dr. Wedell (Institute for Applied Photonics, IAP e.V.) and Dr. M. Haschke (Bruker Nano GmbH, formerly Bruker AXS Microanalysis GmbH) for their help and very important discussions on the experimental results and further applications. The RZPs for the spectrometers were fabricated by the ML&C Company (Dr. A. Weidner). Thanks to Dr. Vladimir Vidal and Heike Löchel at BESSY II for their kind support in the experimental work. Ingo Packe designed and constructed the experimental chamber. Helpful discussions with various specialists from IfG GmbH and IAP e.V. as well as financial support from these organisations are gratefully acknowledged. The financial support from IAP e.V. was made possible through the project IW

A. Erko et al. / Spectrochimica Acta Part B 67 (2012) 57–63

091077 funded by the Federal Ministry of Economy and Technology of the German Bundestag.

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