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ULTRA-SOFT X-RAY EMISSION SPECTROSCOPY A PROGRESS REPORT
J. Nordgren
To cite this version:
J. Nordgren. ULTRA-SOFT X-RAY EMISSION SPECTROSCOPY A PROGRESS REPORT. Jour-
nal de Physique Colloques, 1987, 48 (C9), pp.C8-693-C8-709. �10.1051/jphyscol:19879119�. �jpa-
00227227�
JOURNAL DE PHYSIQUE
Colloque C9, suppl6ment au n012, Tome 48, dgcembre 1987
ULTRA-SOFT X-RAY E M I S S I O N SPECTROSCOPY A PROGRESS REPORT
J. NORDGREN
U p p s a l a U n i v e r s i t y , P h y s i c s D e p a r t m e n t , PO Box 5 3 0 , S-751 21 U p p s a l a , Sweden
Abstract.
A review is presented on recent developments in ultra-soft x-ray emission spectroscopy. Especially, instrumental developments related to high
resolution studies of free molecules and to the use of synchrotron radiation are discussed. The most recent results from three different applications of ultra-soft x-ray emission spectroscopy are presented: electron excited spectra of free molecules, monochromatized synchrotron radiation excited spectra of solids and photon emission following charge transfer in slow ion- atom collisions.
Introduction.
Soft x-ray emission originating in transitions between valence states and core states is of considerable interest for studies of the electronic structure of matter. The binding energy separation of the core states in different atomics species makes soft x-ray spectroscopy a chemically selective method, and since the core states are localized the method is able to probe the local electronic structure. Furthermore, the electric dipole nature of the x-ray transitions allows the angular momentum components of the valence states to be separated. For molecules the selectivity of the soft x-ray transitions is particularly useful in connection with the MO-LCAO description of the molecular orbitals. For solids, on the other hand, the total density of states function (DOS) can be resolved in partial components (PDOS).
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879119
C9-694 JOURNAL DE PHYSIQUE
In the ultra-soft range (below 1keV) emission spectroscopy (USX) has been pursued for the study of free molecules using different means for excitation and wavelength dispersion. High resolution has been achieved by using grazing incidence technique and bright electron excited sources /I/. In low resolution work it has been possible to use photon excitation by means of x- ray tubes 121. In the x-ray range monochromatized synchrotron radiation has been used to excite spectra of free molecules, revealing interesting
phenomena related to the excitation dynamics /3/.
Recent developments in grazing incidence instrumentation /4/ and in synchrotron radiation sources /5/ has opened up the possibility of using narrow band-pass photon excitation for high resolution ultra-soft x-ray emission spectroscopy. The present paper reviews the first results obtained in this new field. It also discusses briefly the most recent progress in electron excited molecular USX spectroscopy. Finally, the application of high performance grazing incidence instrumentation to charge transfer ion-atom collision experiments is discussed and new results are presented.
Ultra-soft x-ray emission spectra o f free molecules.
The MO-LCAO concept (molecular orbitals as linear combinations of atomic orbitals) is particularly useful for interpretation of USX spectra of
molecules. Ultra-soft x-ray transitions connect localized inner holes with outer delocalized molecular orbitals by electric dipole interaction. In a one- center approximation only the dipole allowed components of the final state molecular orbital contribute to the x-ray intensity 161. For symmetric molecules this condition manifests itself as strict selection rules, and in less symmetric molecules x-ray band intensities reflect molecular orbital composition. A number of small molecules (up to the size of aminobenzene) have been studied in high resolution ultra-soft x-ray emission (see reviews in /7/ and 181). Comparisons have been made with theoretical spectra based on intensity models of various degree of sofistication. In particular, the validity of the one-center model has been investigated 191 in order to study its applicability for larger molecules where ab initio methods are not feasible.
The local electronic structure of simple alcohols has been the subject of a recent investigation
/lo/.
Knowledge about orbital composition is valuable for the understanding of bonding mechanisms acting in adsorption on metal surfaces. One question of particular interest is the role of the oxygen 2p lone pair orbital in adsorption processes. Fig.1 shows the oxygen K emission spectra of methanol in comparison with the x-ray spectrum of water. One observes similar spectral features in the two spectra, suggesting that there is a OH x-ray fingerprint not being destroyed when substituting a methylFig.1. Oxygen K emission spectra of gas phase methanol and water. Bars indicate one-center intensities.
I
OXYGEN K EMISSION
METHANOL ::
x: ::.. .
. zr. 4 :
,
$;+. ".,!+:>- ,?,,
,qv* . . :.:$ * ; >,
:? '\ y ,. 3 - ' : * < r &,
.
.
..;.!" %'> . &:. : ;,.G??ri.z;;;2; , !&*:r;< , c,,:;:i7 . : .
:.I C. .i.
..
" ,. ...>. -.<: ..1'"..3 '
WATER
. .
520 525 530 CeVI
b
group
.
This notion is supported by the oxygen K emission spectrum of ethanol 1101. The strong line at about 527 eV, l b l in water and 2a" in methanol, corresponds to the so called oxygen 2p lone pair orbital. Turning to the carbon K spectrum of methanol in fig.2 we observe that this particular band appears with considerable intensity also in this spectrum. This observation, which indicates that there is an appreciable amount of carbon 2p mixed into the "oxygen 2p lone pair" orbital shows a limitation in the lone-pairdescription and brings the attributed role of this orbital for adsorption mechanisms into focus.
-
CARBON K EMISSION
$, 7 ) i ,
r ;
$=.i i >\, ' . !
',
?,,
.f,
. q: v* '?<w*ss.
i
270 275 280 285 C e V l
Fig.2. Carbon K emission spectrum of methanol. Bars indicate one-center intensities.
The attainable resolution in USX spectra of molecules is comparable with the natural lifetime width of the inner vacancy state of the x-ray transitions (0.1
eV).
Hence core state lifetimes can be determined from the measured line widths I111 and fine structure due to vibrational excitations can be studied 1121. Vibrational excitations accompanying inner excitations can be considerable due to the relaxation of the outer electrons and manifestC9-696 JOURNAL DE PHYSIQUE
themselves in the fine structure of the x-ray bands. For instance, a dramatic change in the equilibrium bond length is observed when ionizing a C l s electron in carbon monoxide, while a slight lengthening of the bond is
observed when an oxygen core electron is removed 1131. In the case of carbon dioxide the removal of a C l s electron causes a less dramatic decrease in bond length while oxygen core ionization leads to a substantial displacement of the center atom, thereby breaking the symmetry of the molecule 1141.
The interpretation of USX vibrational fine structure in terms of Franck- Condon overlaps is quite successful. Frank-Condon fits to observed spectra yield information about the potential energy surfaces of the core states and in particular changes in equilibrium bond lengths and geometries upon core excitation and deexcitation 1121. In the x-ray emission spectrum of gaseous acetylene the
Inu
band exhibits well resolved vibrational fine structure, see fig.3. Interpretation is made in terms of multi-dimensional Franck-Condon overlap calculations of core ionization and x-ray deexcitation, and the calculated spectrum is also shown in the figure for comparison 1151.Fig.3. X-ray emission spectrum of C2H2, Inu band. The theoretical spectrum shown to the right is based on a multi-dimensional Franck-Condon calculation.
EXPERIMENTAL
"\
:
J I
.It
$iA
i'
'J!3 2 1 o
, ~ m ~ t ~ l
278 279 280
The finite lifetime of the inner vacancy initial state of the x-ray transitions imposes a limitation to the validity of the Franck-Condon picture. The effect of this limitation is observed in the C K emission spectrum of carbon
monoxide 1131. In order to attain full agreement between theory and experiment the phase of the core state wavefunction has to be considered,
CALCULATED
I ' I ' I ' I
-2.0 -1.0 0.0 1.0
i.e. the interference between excitation-deexcitation channels pertaining to different core vibrational states has to be accounted for.
Satellite lines are frequently appearing in molecular soft x-ray spectra, which are excited with considerable excess energy. A great deal of satellite structure can be rationalized in terms of multiple ionization and excitation upon inner hole formation while, in other instances, rearrangement processes in the x-ray decay are responsible 181. In some cases detailed investigations of the satellite spectra have been carried out based on CI and MC-SCF
calculations of transition energies 116,171. Also, models for simple derivation of satellite energies have been put forward 1181.
Developments in ultra-soft x-ray spectroscopy instrumentation.
The high brightness of the x-ray source required to allow high resolution grazing incidence instruments to be used for studies of free molecules has excluded other means of excitation than electron impact. For solid samples discrete x-ray sources 119,201, bremsstrahlung 1211 and broad band synchrotron radiation 1221 has
been used. The general problem of sample decomposition encountered
when using electron impact ULTRA-SOFT X-RAY EMISSION SPECTROSCOPY
excitation is in practice eliminated FOR FREE MOLECULES
in gas phase studies since sample
gas can be continously replenished Electron
in a windowless differentially
pumped source 1231. e '
A new electron excited source for
x-ray emission studies of gas Differential pumping
samples has recently been
constructed, see fig.5. It operates with a Pierce type electron gun,
provided with an indirect heated Spectrometer
LaB6 cathode. Focussing of the electron beam (typically 100 mA at
10 keV) is made by a magnetic Fig.4. Principle of s e t up for USX quadrupole doublet lens which also s t u d i e s of free molecules.
directs the beam through the entrance slit of a sample gas cell where pressures above 10 mbar are maintained. Three stages of
differential
JOURNAL DE PHYSIQUE
Fig.5. Electron excited x-ray source for gas samples. A: Mechanical pumps (roots- and
turbopumps), B: Sample chamber, C: Collimator, D: Magnetic quadrupole doublet, E: Vacuum valve, F: Electron gun, G: Mass spectrometer for sample decomposition monitoring, H: Translation adjustment for electron gun.
pumping separates the gas cell from the electron gun where pressures below 10-5 mbar are achieved. Also, three-stage differential pumping is employed between the gas cell and the grazing incidence spectrometer to avoid the use of windows. In order to increase the brightness of the source a solenoidal magnetic field is introduced in the excitation region of the gas cell by means of axially symmetric permanent magnets and a soft iron yoke. The electron beam is focussed by being caught by the convergent field which also
increases the path length of the electrons, thereby increasing the excitation efficiency.
Along with the construction of the new source for molecular studies a grazing incidence instrument has been constructed which is portable and has high adaptibility to different sources 141. This design allows us to use also sources at other laboratories, in particular synchrotron radiation sources.
Recent developments in synchrotron radiation techniques have provided very bright sources and for the first time selectively excited high resolution spectra in the ultra-soft x-ray range are possible.
Fig.6. Outline of the fixed-gratings grazing incidence instrument. A: Experiment chamber, B:
Sample, C: Entrance slit, 0: Grating selector, E: Gratings, F: Reference block, G: Optical axis adjustment, H: Rotational feedthroughs, I,J,K: Translational stages, L: Detector, M: Detector house, N: Support structure.
The new grazing incidence instrument has been designed according to a new concept which allows a large spectral range to be covered, still providing high resolution and luminosity. In order to satisfy the requirements of large spectral range and high resolution several gratings are used. The gratings are mounted fixed adjacent to each other on a precision ground reference block.
This arrangement allows the spectrometer to be given a design which easily adapts to different sources since it dispenses with elaborate mechanisms for change of grating, see fig.6. Instead, selection of grating in use is made by operating a movable aperture between the gratings and the entrance slit so that the appropriate grating is illuminated. The fixed grating arrangement allows in particular the incidence angles to be chosen to match the
wavelength regions covered by the different gratings.
Radii of curvature of the gratings, as well as groove densities, may be different which facilitates the accomplishment of optimum performance over the entire spectral region. Obviously, there will be as many focal curves as there are gratings since each grating together with the common entrance
C9-700 JOURNAL DE PHYSIQUE
slit and the (properly positioned) detector forms a Rowland instrument. The detector can be positioned at any point on the different focal curves by means of accurate translation stages, which are driven by computer controlled stepper motors. In particular, the detector can be positioned at off-Rowland positions which is of importance when striving for maximum efficiency at moderate resolution. The reason for this is that the spherical aberration moves the best focus away from the Rowland circle when the grating width is increased.
Detection is made by a large (40 mm) Csl coated multichannel plate (MCP) sensor provided with two-dimensional read-out based on resistive anode technique 1241. Two-dimensional read-out is necessary in order not to impair the resolution due to the curvature of the spectral lines and to account for image distorsion in the detector read-out. The efficiency of the MCP is considerably higher at near normal incidence than at grazing angles.
Therefore the detector can be rotated to a near normal orientation to be used in low resolution-high efficiency mode. In high resolution mode the rotation mechanism is used to orient the detector to be tangential to the focal curve.
A considerable improvement of the efficiency at grazing angles is
accomplished by using a repelling electrode in front of the MCP. It consists of a set of 50 pm wires running parallel with incoming radiation 2 mm above the detector surface and is given a negative potential of several hundred volts. The Csl coating is essential for the efficiency 1251, although it introduces the inconvenience of constantly keeping the detector in vacuum or in dry air due to the hygroscopic properties of this coating material.
The spectrometer design permits spherical gratings to be used without large astigmatic losses in sensitivity. The large imaging detector is instrumental for this achievement which permits high sensitivity and high resolution to be attained simultaneously. The sensitivity depends on wavelength and
resolution selected but as a typical example the high resolution (0.07 eV at 280 eV) sensitivity is This means that 108 photons from the sample are required to yield one detected count (spot size on the sample is assumed to be 0.2 x 5mm2). In low resolution mode (0.6 eV at 280 eV) the sensitivity is 4-1
o - ~ .
Since typical yields for soft x-ray fluorescence are in the order of 1W4 we thus require a flux of exciting particleslphotons per second of 10"to 1013 on the sample. This is easily achieved in a fine focusing electron beam and, as shown below, it can also be attained in a beam of
monochromatized photons at a storage ring.
The resolution of the instrument is set by the spatial resolution of the detector, which is about 60 pm in the direction of dispersion. The
corresponding entrance slit sizes are in the 5-10 pm range. Fig.7 shows the ultimate resolution for a configuration of two 5 m gratings and one 3 m
PHOTON ENERGY < e V )
Fig.7. Ultimate resolution of multi-grating grazing incidence instrument for a partcular configuration of gratings.
grating, with groove densities 1200, 400 and 300 mm-1, respectively.
Ultra-soft x-ray spectroscopy using synchrotron radiation.
The use of synchrotron radiation for excitation of soft x-ray spectra has several advantages. Synchrotron radiation sources are very bright compared to other sources and, as will be demonstrated below, they even allow primary monochromators to be used to obtain selectivity in the excitation. By using narrow band-pass excitation
overlaying satellite lines can be ULTRA-SOFT X-RAY EMISSION SPECTROSCOPY
isolated and accidental occurrence of USING SYNCHROTRON RADIATION
irrelevant transitions (like inner-core transitions and higher order spectra) eliminated. Also, threshold ~henofnena and resonant behaviour can 'be studied in detail. Another aspect of importance is that of radiation damage 1261.
Chemically fragile samples are more likely to survive the dose required in exposure to monochromatized exciting photons than in broad band excitation.
The first series of experiments in Grazing Spectrometer Incidence
,
hV GSamplem cham^^
ultra-soft x-ray emission using
C9-702 JOURNAL DE PHYSIQUE
monochromatized synchrotron radiation were carried out recently at the synchrotron radiation laboratory (HASYLAB) at DESY in Hamburg. The experiment was set up at the FLIPPER I multipole wiggler beam-line. The FLIPPER I monochromator employs a plane grating for dispersion and a parabolic mirror for focussing on the exit slit. A set of premirrors at various incidence angles are available to suppress higher order radiation. Measured output lies in the
l o 1
2-101 s - I range at resolutions between 0.5 eV and 10 eV in the 20 eV to 1 keV photon energy range 151. As discussed above this photon flux is of the required magnitude to allow fluorescence spectra to be recorded in high resolution. In fig.8 is shown a photograph of the new grazing incidence instrument attached to the experiment chamber at the FLIPPER I station.monochromatized synchrotron radiation.
In the following we present briefly some of the results obtained in the first experiments. Reference to papers presenting more thorough discussions are given passing the different subjects.
In the ultra-soft x-ray emission spectrum of TIN part of the Ti L emission (3s-2p) overlaps the N K emission. This has presented problems for x-ray band structure investigations of this compound since the separation of the Ti L spectrum requires the excitation energy to lie between the thresholds for N l s and Ti2p, a restriction that has not been met in previous work. Attempts to overcome this difficulty have been made by subtracting the known 3s-2p
line shape in T i 0 1271 or by studying ZrN instead, assuming similar electronic structure /28/.
We have recorded the N K emission spectrum of TIN using
monochromatized synchrotron radiation for excitation, thereby obtaining the pure spectrum. In addition, by tuning the excitation energy down to near threshold we can remove satellite structure appearing at excess energy
excitation to obtain the undistorted band spectrum mapping the N2p contribution to the valence band, see fig.9a. The spectrum has been
aligned to the Fermi energy using XPS binding energy data.
In fig.9b is shown the Ti Llll emission spectrum obtained by tuning the excitation energy to 458 eV, i.e. below the LII threshold, and aligned by means of XPS data.
Comparing the
N
K and Ti Llll spectraSOFT X-RAY
EMISSION FROM TiN
c:Ti L
h v . , , = 4 8 0 , 4 5 8 e V D i f f e r e n c e
Spectrum
b:Ti L
-15 - 1 0 -5 E, 5
E n e r g y ( e V )
gives a complementary picture of the valence band. The results support
band structure calculations Fig.9. Nitrogen K and Titanium L emission describing the outermost band as spectra of TiN.
mainly Ti3d and the 5 eV band as N2p with a significant contribution of titanium d electrons 1291.
F i g . 9 ~ shows the difference between Ti L emission spectra recorded at energies well above and below the LII threshold. The resulting spectrum, which presumably would show the Ll( emission, does not resemble the Llll spectrum as expected. Also, the intensity of the difference spectrum is much too high to be in concordance with the LII Coster-Kronig depopulation rate 1301. In order to explain these observations we turn to fig.10, which shows a series of Ti spectra recorded at various excitation energies. We see that the spectrum reveals a resonant behaviour when the excitation energy passes through the 460-470 eV region, with a maximum intensity of the 460 eV line at 465 eV excitation energy. We explain the observed resonance as the decay of an atomic like l.111-3d state populated via Coster-Kronig decay of an Lll-3d
C9-704 JOURNAL DE PHYSIQUE
state. The absence of the normal Ti LII spectrum is explained by the strong Coster-Kronig decay of the LII vacancy. A more detailed discussion of the present results can be found in ref.1311.
The usability of selective excitation for soft x-ray spectroscopy is also demonstrated in the L emission spectrum of Cu, shown in fig.11 1321. The spectrum excited with 970 eV photons shows two bands corresponding to a band electron filling a 2 ~ 1 1 2 Or 2P312 vacancy, respectively.
PHOTON EXCITED Ti L EMISSION FROM TiN
I n t e n S i t Y
4 . . .
Emission Energy (eV)
Fig.10. Titanium L emission spectra of TIN excited at various photon excitation energies.
I ,
905.0 915.0 925.0 935. 0 945. 0 955.0
Energy (eV)
Fig.11. Cu LII,III emission spectra of copper metal excited at different photon excitation energies.
130
-
110-
Intensity Copper Cu L ernissi
90
-
Exc. Energy : 970 eV935 eV
70
-
Difference Spectrum50- 30- 10-
Coster-Kronig decay of LII holes and shake-off processes in Llll hole formation are expected to give rise to satellites superimposed on the Llll band. Exciting the emission below the LII threshold and near the Llll
threshold removes the LII line and the satellites and it allows the clean Llll bandspectrum to appear. The relative intensities of the LII and Llll lines and the satellite contribution then yield information about the strengths of the
non-radiative decay channels and about the shake-off probability.
In an investigation of the copper L and oxygen K emission spectra of high Tc
superconductors 1331 the same technique as for copper metal was employed. Spectra were recorded at excess excitation energies and near thresholds allowing the satellite structure to be separated from the diagram spectra. Fig.12 shows the Cu Llll and 0 K emission spectra of YBa2Cu307 recorded at near threshold energies and aligned to the Fermi level by means of photoemission and x- ray absorption data 1341. The near threshold excited spectra are supposedly free from satellites and thus map the partial density of states of Cu3d and 02p character,
respectively. Apart from enabling the decomposition of the total DOS the present method provides another important quality in that it probes the electronic structure of the
bulk of the material. This is particularly important for the ceramic superconductors which are known to change surface
composition in vacuum.
SOFT X-RAY EMISSION SPECTRA OF YBa2Cu301.,
Energy (eV)
Fig.12. Copper Llll and oxygen K emission spectra of a high Tc superconductor.
Ultra-soft x-ray emission i n charge transfer collisions.
Laboratory studies of the interactions of highly charged ions play an
important role for the understanding of plasmas. While hot plasmas like the solar corona is dominated by electron impact excitation certain cold plasmas in space may critically depend on charge transfer to radiating excited states of highly charged ions. In fusion plasmas both types of processes can occur.
Diagnosis of plasmas is commonly made by studying the photon emission spectrum to obtain level population rates which reflect the dynamics of the plasma.
C9-706 JOURNAL DE PHYSIQUE
In recent experiments we have
ULTRA-SOFT X-RAY EMISSION SPECTROSCOPY
applied high resolution ultra-soft FOR STUDIES OF CHARGE TRANSFER COLLISIONS
x-ray emission spectroscopy to the study of charge exchange in slow ion-atom collisions. The work was made at the Minimafios ECR ion source at the LAGRIPPA-CEA-CNRS facility in Grenoble 1351. A charge specific beam of highly charged ions, produced in the ECR source and purified in a double separator magnet was brought to collide with a target of helium or molecular hydrogen at pressures in the 5- 10.1
o - ~
mbar range. Photon emission in the 20-250A
region subsequent to electron capture in the collision was recorded using the new portable grazing incidence instrumentdescribed above 141.
Systems studied were He- - 0 m CHARGE TRANSFER COLLISIONS like N, 0, F and Ne ions
colliding on He and H2 targets at velocities near 1 a.u. Experiments of this kind have been made previously by several workers, see e.g.
/36,37,38/. However, as will be demonstrated below the performance of the new grazing incidence
instrument allows spectra to be recorded with considerably higher
resolution and quality than before revealing new information.
In
fig.13 are shown emission sDectra in the100-200
A
' range recorded1
Wavelength (A) in collisions between 6OkeV06+ ions and helium Fig.13. Photon emission following electron capture in 60 keV 06+
collisions with helium and molecular hydrogen.
respectively molecular hydrogen. The insets are plotted with enlarged scales to bring out the low intensity lines. Both spectra show a large number of peaks which have not been observed in a pure collision experiment before.
The intense peaks, seen already in the non-magnified spectrum, mostly correspond to single capture processes. The difference in the intensity distributions between helium and hydrogen reflects the capture cross section dependence on the target electron binding energy: the less tightly bound hydrogen electrons are more likely to populate n=4 states while the helium I s electrons are dominantly captured in n=3 1391. We have determined the single capture cross sections from recorded line intensities by correcting for detection efficiency 1401 and accounting for branching ratios 1411. The results for He are shown in table 1 together with previous experimental determinations f371 based on measurements partly in other wavelengths and calculated cross sections 1421.
Table 1. Single capture cross sections for 60 keV 06+ on He.
Present work Previous exp. Calculations
1371 1421
State
Single captu,re to metastable ions are also observed in a number of cases, giving rise to transitions in the quartet system. A few of these lines have been found to correspond to non-core conserving capture, i.e. excitation of the core in the capture process.
A number of lines, observed predominantly in collisions with helium, have been identified as transitions in 04+, indicating double capture processes.
Some of the lines correspond to double capture to states involving high quantum numbers, e.g. 2s6f, with'very low cross sections for single capture.
This is indicative of strong electron correlation in the capture process.
Evidence of correlated double electron capture to autoionizing states has been observed in Coster-Kronig spectra 143,441, but to our knowledge this is the first observation in photon emission.
C9-708 JOURNAL DE PHYSIQUE
Conclusions.
A progress report is given on ultra-soft x-ray spectroscopic studies of molecules and solids. In particular, recent instrumental developments are discussed. For the first time monochromatized synchrotron radiation has been used to excite ultra-soft x-ray emission spectra, recorded in high resolution. This achievement opens up a new source of information in x-ray spectroscopy, and several advantages with the technique are demonstrated by the results presented. The application of high performance grazing incidence technique to the study of charge transfer in ion-atom collisions is also discussed. It is demonstrated that substantial improvements in spectral quality are possible, and new results from charge transfer studies are presented.
Acknowledgement.
This work was supported by the Swedish Natural Science Research Council.
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