core-level nonlinear spectroscopy triggered by stochastic...
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ARTICLE
Core-level nonlinear spectroscopy triggeredby stochastic X-ray pulsesYves Kayser 1,2*, Chris Milne 2, Pavle Juranić2, Leonardo Sala2, Joanna Czapla-Masztafiak3, Rolf Follath 2,
Matjaž Kavčič4, Gregor Knopp 2, Jens Rehanek 2,8, Wojciech Błachucki 5, Mickaël G. Delcey6,
Marcus Lundberg 6, Krzysztof Tyrała 3, Diling Zhu7, Roberto Alonso-Mori 7, Rafael Abela 2,
Jacinto Sá 5,6* & Jakub Szlachetko 3*
Stochastic processes are highly relevant in research fields as different as neuroscience,
economy, ecology, chemistry, and fundamental physics. However, due to their intrinsic
unpredictability, stochastic mechanisms are very challenging for any kind of investigations
and practical applications. Here we report the deliberate use of stochastic X-ray pulses in
two-dimensional spectroscopy to the simultaneous mapping of unoccupied and occupied
electronic states of atoms in a regime where the opacity and transparency properties of
matter are subject to the incident intensity and photon energy. A readily transferable matrix
formalism is presented to extract the electronic states from a dataset measured with the
monitored input from a stochastic excitation source. The presented formalism enables
investigations of the response of the electronic structure to irradiation with intense X-ray
pulses while the time structure of the incident pulses is preserved.
https://doi.org/10.1038/s41467-019-12717-1 OPEN
1 Physikalisch-Technische Bundesanstalt, Abbestr. 2-12, 10587 Berlin, Germany. 2 Paul Scherrer Institut, 5232 Villigen PSI, Switzerland. 3 Institute of NuclearPhysics Polish Academy of Sciences, 31–342 Krakow, Poland. 4 Institut Jožef Stefan, Jamova 39, 1000 Ljubljana, Slovenia. 5 Institute of Physical Chemistry,Polish Academy of Sciences, 01–224 Warsaw, Poland. 6Department of Chemistry- Ångström, Uppsala University, 751 20 Uppsala, Sweden. 7 Linac CoherentLight Source (LCLS), SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA. 8Present address: Advanced Accelerator Technologies AG, 5234Villigen PSI, Switzerland. *email: [email protected]; [email protected]; [email protected]
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Stochastic processes, starting from simple Bernoulli andrandom walk mechanisms, mathematically describe evol-ving systems in different research fields like biology,
chemistry, ecology or economy on the basis of random variables.In the field of X-ray physics, stochastic processes are mostlystudied in the field of astrophysics when investigating the char-acteristics and properties of X-ray bursts from micro-quasars,black holes or the collapse of massive stars1,2. At the mostsophisticated X-ray facilities, the X-ray free electron laser (XFEL)facilities3, the X-ray pulse generation is often based on a sto-chastic process, called self-amplified spontaneous emission(SASE). The process of spontaneous emission from undulatorswas proposed theoretically in the 1980s4 but demonstrated only20 years later5. While the stochastic nature of SASE radiation iswell understood and can be treated with physical models6, therandom nature of the generated X-ray pulses leads to largeuncertainties in time, space, intensity and energy and has com-pelled different instrumental developments for diagnostics7,8
while still imposing restrictions. Experimental developments werespurred as well and led to pioneering experiments regardingtiming9 and stimulated emission10.
Since the advent of XFELs, X-ray spectroscopy was mainlydedicated to either studies of the unoccupied electronic statesusing X-ray absorption spectroscopy (XAS), or investigations ofthe occupied electronic states by means of X-ray emission spec-troscopy (XES). Due to the mentioned stochastic nature ofXFELs, XAS experiments were either realized in a scanning-basedapproach with a monochromatized XFEL beam11,12 or using abeam splitter13,14, a seeded XFEL beam15,16 and a transmissivegeometry17,18, respectively. However, due to limited diffractionefficiencies in the beam-splitting or in the monochromaticapproach only a small fraction of the incident photons is effec-tively used and nonlinear studies are impeded because of reducedintensities on the sample. Furthermore, the use of a mono-chromator extends the pulse duration significantly19,20. Conse-quently, the pulse duration of monochromatized radiation will befar above the lifetime of core level vacancies and the effectivelyachievable time resolution is limited. XES experiments wererealized in the non-resonant regime far above the core-levelionization threshold where the stochastic nature of the incidentenergy distribution was less detrimental compared to XASexperiments21–23.
In this work a reconstruction methodology is introduced andused to demonstrate how the stochastic nature of SASE incombination with non-invasive diagnostics, in practice a trans-missive spectrometer which does not affect the properties of theSASE pulses, can be used to map simultaneously the unoccupiedand occupied electronic states of a scattering atom exposed toshort and intense X-ray pulses. The core concept consists of usingthe correlation between the spectral distributions of the individualincident, non-monochromatized SASE pulses and each corre-sponding XES signal from the sample for a series of 103–104
pulses. It is shown that nonlinear X-ray processes in the vicinityof an ionization threshold can be investigated, allowing theexpansion of the scope of two-dimensional spectroscopy techni-ques available at XFELs beyond the applications of resonant X-ray emission spectroscopy (RXES)24 at synchrotron radiationfacilities. The discrimination of intensity-induced X-ray trans-parency towards opacity due to a few-eV variation of the incidentphoton energy will be discussed in this manuscript and attributedto the sequential ionization and excitation of atomic states withfemtosecond lifetimes. The presented methodology uses SASEradiation intrinsic properties to measure the unoccupied andoccupied electronic states of atoms. Effective utlilization of themost intense and, even more important, the shortest pulsedurations available, is enabled and ultimately nonlinear studies on
the electronic structure of a system can be realized. Hence, anattractive and possibly unique approach towards attosecond X-ray science in the vicinity of ionization thresholds is presented.
ResultsExperimental Setup. The sample was an aqueous dispersion of30 nm in diameter Fe2O3 nanoparticles which was replenishedbetween successive XFEL pulses by means of a liquid jet setup.The spectral distribution of the individual SASE pulses wasmonitored using a transmissive bent crystal spectrometer with aspectral resolution of about 0.1 eV25 and the corresponding X-rayemission spectra of the measured Fe Kα and Kβ emission lineswere recorded on a pulse-by-pulse basis using two von Hamoscurved crystal X-ray spectrometers installed in a horizontal dis-persion geometry26, as shown in Fig. 1. The introduction ofdispersive X-ray spectrometers has proven to be essential sincethey allow for the acquisition of complete X-ray emission spectraat the full XFEL repetition rate. A Be lens stack, installeddownstream of the transmissive spectrometer, was used to focusthe SASE pulses on the sample. Thus, the XES source size, definedby the overlap of the incident photons with the sample volume,was small enough to preserve the intrinsic sub-eV energy reso-lution of the von Hamos spectrometers used. Indeed, the fullpotential of RXES experiments is provided when the experimentalresolution is lower than the line-width broadening induced by thenatural lifetime of the atomic states involved in the transition.
Principle for the reconstruction of two-dimensional spectro-scopy maps. At a hard XFEL facility each single-shot XES spec-trum corresponds to the weighted sum of XES spectra producedwith perfectly monochromatic incident X-ray beams of differentphoton energies. The relative contribution to the measuredintensity is given by the number of photons in each monitoredincident energy channel. The stochastic nature of the XFELsource results in intensity and mean photon energy fluctuationsas well as spectral modes at different photon energies for eachindividual pulse. Therefore, the spectral distributions of eachSASE pulse and consequently of each XES spectrum do not onlyvary on a pulse-to-pulse basis but are unique and cannot inprinciple be reproduced by a linear combination of a number ofother pulses. Further, the spectral distributions of the incidentSASE pulses and the XES signals are correlated through a two-dimensional matrix describing the population of the occupiedand unoccupied electronic states as a function of the incident andemission energy.
Matrix formalism. From the synchronously recorded single-pulse maps of the incident broadband SASE pulse spectral dis-tribution and the corresponding XES spectra, both measured inhigh-energy resolution, the RXES maps were reconstructed, asdisplayed in Fig. 2, through a matrix formalism using theintensity values of the SASE spectra and of the XES spectra as thematrix elements. One dimension of the SASE and XES matrices,labeled hereafter Jkm and Skn, corresponds to the pulse number(1…k) and the second one to the incident and emission energychannels, respectively, where 1…m and 1…n represent therespective energy channels discriminated by the spectrometers.The RXES matrix Rmn, describing the dependence of the spectraldistribution of the emitted photons on the incident photonenergy under resonance conditions, is obtained by first calculat-ing the Moore–Penrose pseudoinverse matrix Jkm+27 and multi-plying it by Skn, Rmn= Jkm+ ∙ Skn as described in the Methodssection. Thanks to the intrinsic stochasticity of the SASE pulses,the pseudoinverse matrix Jkm+ exists. However, due to theinherent experimental noise limiting the numerical solvability,
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the pseudoinverse matrix Jkm+ was calculated using compactsingle value decomposition. The RXES matrix Rmn was recon-structed column-wise using Tikhonov regularization28, with theregularization parameter being determined by means of the L-curve29. Furthermore it is worthwhile to note that X-rayabsorption curves can also be reconstructed using the samemethodology when monitoring only the X-ray fluorescenceintensity emitted from the sample, Rm1= Jkm+ ∙ Sk1.
Reconstructed RXES maps. An example of a reconstructed RXESmap Rmn for the Kβ line is shown in Fig. 2 together with therecorded single-shot series of SASE (Jkm) and XES (Skn) spectrafor a total of 60,000 pulses. A qualitative validation of thereconstructed RXES maps is provided in Supplementary Fig. 1using synchrotron radiation data for the identical sample. It hasto be noted that the main absorption is reproduced at the correctposition and with an identical slope than the synchrotron datawhile the weak pre-edge feature contained in the synchrotronRXES data is not included in the XFEL reconstructed RXES. Theproneness of the reconstruction methodology to numerical errors,intrinsic to matrix inversions, and the ensuing need for numericaltools, for example regularization methods, are the likely reasonsfor the absence of the pre-edge feature. This issue in the recon-struction of low intensity features can be improved in future bybetter measurement statistics when using more efficient andpossibly background-free spectrometers and detectors to improvethe signal-to-noise ratios or possibly also by using a differenttuning of the SASE parameters.
The standard requirement of RXES experiments for incidentradiation with a narrow energy bandwidth is relaxed andthroughout the experiment no scanning components arerequired. Hence not only dead-time is eliminated, but over-coming the necessity for a monochromator allows avoiding theexpansion of the pulse duration and the rejection of the photonsnot fulfilling the Bragg condition. Intensity-dependent studies
using the currently shortest available X-ray pulses, which are ofabout the same femtosecond-duration as the lifetime of thecreated core-level vacancy, are only enabled through thereconstruction of RXES maps. In the described approach it ismandatory that all the photons contained within an individualSASE pulse are recorded by the transmissive spectrometer inorder to not neglect any contribution to the production of theXES signal in the reconstruction methodology. The RXES mapitself is only reconstructed on a narrower incident energy rangefrom 7105 eV to 7140 eV since the low photon intensity regionson the low- and high-energy side of the mean SASE spectrum arecut-out in the RXES map Rmn using a box filter (marked inFig. 2). The energy of the incident X-ray pulses was broad enoughto cover the resonance regime in the vicinity of the ionizationthreshold and allowed acquiring a complete picture of theelectronic density-of-states (DOS). A series of 1200 individualXES spectra, corresponding to an acquisition time of 10 s giventhe 120 Hz repetition rate of the LCLS, that is on time scaleswhich are not achievable when using standard monochromatorunits to scan the incident photon energy and recording thefluorescence intensity11,30, provides together with the spectraldistribution of the corresponding SASE pulses all the informationcontained in the RXES map (Supplementary Fig. 2). Indeed, it isreported that between 200 and 1000 pulses are required for aproper intensity normalization of the measured fluorescencesignal per measurement point in the scanned incident photonenergy range11,30. Hence, based on a conservative estimation andconsidering dead time due to the necessary monochromatormovement, the gain in efficiency provided by our approach is atleast an order of magnitude when covering the same incidentphoton energy window in steps of 1 eV.
The XES spectrum and the XAS signal can be obtained fromthe reconstructed RXES map by projecting the average intensityfrom selected regions of interest onto the respective energy axis.The X-ray emission spectrum shows the Kβ line which originatesfrom metal 3p to 1s electron transitions, as well as the two orders
Dispersion
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Fig. 1 Experimental setup allowing for the use of stochastic X-ray pulses in two-dimensional spectroscopy. The spectral distribution of the incident photonswas detected using a thin crystal transmissive spectrometer before they were focused by a movable Be lens stack on the liquid jet sample. The spectraldistribution of the emitted X-ray photons was monitored for two X-ray emission lines with one dedicated von Hamos spectrometer, respectively. Allspectrometers allow for a single-shot collection of the X-ray spectra with high energy resolution as is shown in the displayed spectra which were acquiredfor a single SASE pulse (insets)
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of magnitude weaker valence-to-core (V2C) line, associated toelectron transitions from valence or ligand orbitals to the metallic1s state. The Kβ mainline is sensitive to the metal spin state andthe metal-ligand covalency while the V2C region containsinformation about the ligand identity, electronic structure andmetal-ligand bond length31. A calculation of the Kβ spectrum ofFe2O3 using the crystal field multiplet model (see SupplementaryNote 1) gives a good overall agreement with the measuredspectrum, although the splitting between Kβ1,3 and Kβ′ is toolarge due to an overestimation of the 3p-3d exchange in theatomic model.
The K-edge XAS obtained from the RXES map deliversinformation on electron transitions from the core orbital to theconduction band and possible pre-edge features are assigned toquadrupole transitions to empty metallic 3d states. The RXESreconstruction methodology offers a potential way to simulta-neously perform XES and XAS experiments, which correspondsto a simultaneous mapping of the unoccupied and occupiedelectronic states, in order to follow electronic transitions with atemporal resolution solely restricted to the pulse duration.
Indeed, the most crucial aspect of the proposed methodology isthat the temporal broadening related to the monochromatizationof the X-ray pulses is avoided. The temporal broadening ofmonochromatized XFEL pulses is related to the extinction lengthof the X-rays in the monochromator crystal. For X-rays with aphoton energy of a few keV, the extinction length amounts to fewmicrometers and may lead to pulse broadening of up to 20–40 fs,
setting thus the physical limit for ultrafast time-resolvedexperiments19,20. A RXES map reconstructed for 5 fs-durationSASE pulses incident on the sample is shown in SupplementaryFig. 3. Regarding the short lifetimes (femtoseconds) of the excitedelectronic states and rearrangement processes, the ability to usethe shortest available X-ray pulses is of prime importance forspectroscopy studies and currently, efforts are being made todeliver high-power and short-duration X-ray pulses at XFELfacilities32,33. While the impact of a monochromator on the pulseduration can be bypassed, any fundamental limits of the machinein terms of pulse duration cannot be coped with when using thereconstruction methodology.
Incident X-ray intensity dependent studies. As a first applica-tion of the reconstruction methodology, we opted to investigateX-ray matter interactions when increasing the incident beanintensity towards the nonlinear excitation regime in which multi-photon-in and multi-photon-out scattering processes take placeand unique configurations of atomic charge states with femto-second long lifetimes are created. The detailed information on theelectronic configuration of atoms provided by RXES is necessaryto acquire a complete picture of the mechanisms driving thenonlinear interaction in the vicinity of a core-level ionizationthreshold. It should be noted, that the reconstruction metho-dology does not require a priori knowledge of the physical pro-cesses involved in the X-ray matter interaction and theirdependence on the total incident X-ray intensity. Only the
XES projection
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Fig. 2 Example of the reconstruction of the two-dimensional spectroscopy map of the Kβ line. Using the spectral distribution of 60,000 incident SASEpulses a and of the corresponding XES produced by the broadband incident radiation b, the RXES map can be successfully reconstructed c using a matrixinversion formalism as indicated by the elastic scattering feature in the map and the XES d, which is compared to a multiplet calculation, and XAS spectrawhich can be extracted e. This methodology alleviates the requirement for monochromatic radiation to measure the RXES map allowing for increasedefficiency of such measurements
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contributions at different photon energies are considered torecover the response matrix of the system, the RXES map, whichdescribes the response of the electronic structure as a dependenceon the incident photon energy in the vicinity of an ionizationthreshold. Considering RXES maps reconstructed for differenttotal incident intensities, it is shown hereafter that the recon-struction methodology enabled the simultaneous study of twonon-linear X-ray processes, namely saturable absorption (SA) andtwo photon absorption (TPA). In the present experiment theintegral intensity incident on the sample was increased from2.2 × 1015W/cm2 to 1.3 × 1017W/cm2. The RXES maps obtainedfor the highest X-ray intensity is plotted in Fig. 3 together withthe difference map with respect to the RXES map reconstructedfor the lowest incident X-ray intensity displayed in Fig. 2. Energyshifts along the incident energy axis as well as along the emissionenergy axis are observed in the difference map.
The changes in the line shape and position of the XESprojections indicate an X-ray induced relative population increaseof low-spin states with respect to high-spin states. Due to theinteraction between the 3p and 3d electronic levels, the Kβ1,3main line presents a sensitive probe of the Fe spin state and wasused as such for characterizing the spin crossover mechan-ism22,34. The switching in the spin state results from excited statesand the RXES reconstruction methodology can be used in the
future as a tool for studying excitation and relaxation mechan-isms and their interplay with structural changes. Indeed, thereconstructed RXES maps for different incident intensitiesprovide more sensitive information than non-resonant XES aloneas shown best by the XES projections for different photonenergies plotted in Fig. 4A. It becomes evident that in theresonant regime an improved relative sensitivity towards the spinstate, and a better discrimination, can be achieved by anoptimized choice of the incident photon energy. This choice isonly rendered possible by the complete picture provided by theRXES maps which contain all relevant fingerprints on changesinduced in the occupied and unoccupied electronic states.
Simultaneously, the projection of the XAS data, displayed inFig. 4B, shows a blueshift of the inflection point in the absorptioncurve by about 2 eV, changes in the absorption cross-section inthe below and in the above edge region, reflected through changesin the XES intensity, and a blurring of the edge position, madeobvious by a 30% increase in the peak width of the derivative ofthe XAS curve. The spectral differences do not reveal thecontribution of resonances with possibly depopulated valencestates. The photoelectric absorption cross-section σk wereestablished using synchrotron radiation data (cold spectrum)and referencing the XAS curve extracted from the reconstructedRXES map recorded at the lowest intensity to it, assuming thatthe incident photon energy dependence is identical.
For incident energies below the ionization threshold, theincrease in the ionization cross-sections (decreasing X-raytransparency) with increasing incident X-ray intensity can beexplained through two photon absorption (TPA). The atom isionized via a sequential two-step process during which the 1selectron is pumped to an intermediate state followed by a secondabsorption process. The TPA cross-section is equal to the productof the cross-sections of each photon absorption event and thelifetime of the intermediate state. In the vicinity of an ionizationthreshold, the TPA mechanism may be mediated by intermediateatomic resonances35, as well as enhanced by higher probabilityfor two-photon absorption via a virtual intermediate state 36. Todetermine the cross-sections for TPA, the measured XES yieldsfor different incident X-ray intensities at an incident X-ray energyof 7118 eV were transformed to total X-ray rates as displayed inFig. 4C. The data shows as expected a quadratic dependence andthe TPA cross-section σTPA at 7118 eV was determined to be3.4 × 10–54 cm4 s which compares reasonably well to the expectedvalue of 1.2 × 10–54 cm4 s when considering a Z−4 dependence forTPA36.
The overall intensity decrease in the XES signal is connected toa corresponding decrease in the absorption cross-section forincident energies above the ionization threshold, displayedexemplarily for an incident energy of 7133 eV in Fig. 4D. Thisbehavior is characteristic for SA, comparable to the few reports inthe literature on nonlinear X-ray matter interaction measured intransmission experiments in the vicinity of the respective core-level ionization threshold17,37,38. From the measured data, the K-shell production rate was deduced and a saturation intensity Isatof 7.7 × 1017W/cm2 was determined using a simple model for SAin which it is assumed that the relaxation rate of the excited statesis independent of the incident intensity39.
The experimental observations on the intensity-dependence ofthe SA and the TPA were compared to calculations based ontime-dependent rate equations (Supplementary Note 2) andMonte Carlo simulations (Supplementary Note 3). For SA a two-level system with a cross-section of 6.5 × 10–20 cm240 wasassumed for establishing the rate equations while for TPA, inagreement with the sequential excitation mechanism, a three-levelsystem with an intermediate virtual state was used. A virtualintermediate state was considered rather than a real intermediate
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Fig. 3 Reconstructed RXES maps for different incident intensities. Thereconstructed RXES maps for the highest average X-ray intensity of theSASE pulses on the sample a and the difference map b towards themeasurement at the lowest intensity, that is the RXES map from Fig. 2,c, are shown. In the vicinity of the core shell ionization threshold noticeablechanges, with relative intensity differences of up to +17% and −32% canbe observed. Notably a shift in the 1s ionization threshold and a change inthe intensity ratio between the Kβ1,3 and the Kβ′ emission lines,representative for the 3p-3d spin-coupling splitting, can be observed
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state involving the valence shells based on the consideration thatthe Kβ XES line does not provide direct evidence forthe contribution of valence holes. Furthermore, the shift in thebinding energy of the remaining electrons can be expected to besmaller than for the shift observed in Fig. 4B of the core-levelelectrons35. The participation of valence states in the describedprocess cannot be excluded, but virtual intermediate states areconsidered to be more likely. Moreover, the basic principleunderlying the mechanism for sequential TPA would be identical.For the three-level system with a virtual intermediate state across-section of 4.2 × 10–21 cm241 for the first excitation step, alifetime of the intermediate state of 0.11 fs42 and a cross-sectionevent of 1.3 × 10–17 cm236 for the second absorption wereassumed. In both processes, TPA and SA, the final state isidentical and has an electronic configuration has a vacancy in the1s shell which decays afterwards through a series of X-ray andAuger cascades with intermediate states having sub-femtosecondlifetimes. The cascade series result in charged atomic statescharacterized by vacancies in the inner- and outmost electroniclevels and as a consequence an increased 1s ionization thresholdas compared to the ground state43. Monte-Carlo simulations wereused to follow the transfer mechanisms of the vacancy after a 1sionization event and indicated, as shown in Fig. 5, that thetransfer of the vacancy in the electronic configuration to thevalence shell takes at most 3–4 fs after which the atom is mostlikely in a 3+ or 4+ valence state. As a consequence theionization threshold for the 1s level increases by more than 40 eV,respectively more than 60 eV for the 3+ and 4+ valence states.
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Fig. 4 K-edge XAS and Kβ XES data showing the SA response and the increased below-edge opacity. The XES signal a reveals a change in the spinstate of Fe2O3 with increasing incident X-ray intensity whereas the XAS behavior b exhibits an increased opacity below and higher transparency abovethe ionization threshold. Further, the TPA cross-sections for a below ionization threshold energy at 7118 eV c and the saturation intensity above theionization threshold at 7133 eV d were derived from the recorded incident photon-energy dependent data on the K-shell production rate, respectivelythe photoionization cross-section. The experimental data are plotted along with the theoretical functions (red solid lines) and the rate equationcalculations for both smooth Gaussian distribution (solid lines) and a spiky pulse structure (green dashed line) as well as the Xmdyn calculations on SA(green dots)
[Ar]
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Final electronic configurationIncrease in ionization energy with respect to [Ar] 3d64s2
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Fig. 5Monte Carlo simulation of the decay processes following the creationof a vacancy in the 1s shell. Monte Carlo simulations were performed tofollow the transfer of the created vacancy at the 1s level due to radiative andAuger transitions. The simulation was performed for atomic Fe and revealsthe likelihood distribution of the different valence state. The time neededfor the decay cascade was also followed and was found to be considerablyshorter than the duration of the SASE pulse. The shift in ionization energyfor the different valence states of atomic Fe was calculated usingXATOM46. It follows that for the higher valence states the 1s electronscannot be directly excited to the continuum and less likely via TPA for theincident photon energies used
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The calculated pronounced dependence of the ionization thresh-old on the charge state of the Fe atom is in agreement withexisting experimental literature data on Fe44 and observations atXFELs on Al43. Due to this shift of the energy levels to highervalues, multiply charged states are not accessible in the presentexperiment. Furthermore, solid-state effects, for example, changeof atomic positions to account for charge state changes orscreening effects, are considered to have only a secondaryinfluence since the time domain probed in the experiment islimited by the pulse duration of 35 fs. Still, these effects should beconfirmed experimentally.
When considering the incident photon energy distribution, afurther ionization via an excitation of the remaining 1s electroncan only occur through a TPA process which is less likely by afactor of 40 because of the larger energy difference to theionization threshold compared to the ground state configurationof the scattering atom36,41. In general, the change of the valencestate from 1+ to N+ and the associated increase of the ionizationthreshold by 10 eV to 160 eV implies that further core-levelionization events can be neglected until the scattering atomreturns to ground state configuration (1s22s22p63s23p63d5 for FeIII) by electron capture processes. At the same time, thelikelihood of incident X-ray photons finding an atom in a highlyionized state increases with increasing intensity.
The observed saturation at high field intensity was qualitativelyreproduced using Xmdyn calculations for atomic Fe (Supple-mentary Note 4)45–47. The results of the simulation are shown inFig. 4D. Those calculations confirmed the above predictions,namely that once a vacancy is created in the 1s shell the ionizationenergy shifts due to an increase in the valence state during thetransfer of the vacancy to the valence shells, as displayed in Fig. 5,and hence the atom cannot reabsorb again. It is interesting tonote that the experimental data could be interpreted satisfactorilywhile considering the case of an isolated atom in theory. Since theincident photon energies vary in the vicinity of the Fe 1sionization threshold, mainly the ground-state population isinvestigated. For this reason, post-ionization processes likeelectron-ion collisions43 or charge transfers48 can be disregardedas prime contributors to the observed changes. Finally, it can beconcluded that the selected approach to use tabulated atomiccross-sections, thus the isolated atom approach, is in firstapproximation sufficient to describe the experimental data. Thisobservation is corroborated by the relatively good agreementbetween the dependence of the changes on the increasing incidentX-ray intensity expected in theory and observed in theexperimental data.
Based on the time-dependent rate equations the time needed toreturn to the electronic configuration of the ground state wasinvestigated. Information on this time-lapse is very scarce in theliterature, mostly due to the lack of adequate experimentalmethods. This is further hindered by the complexity of thevacancy transfer processes that are associated via radiative andnon-radiative decays from higher atomic levels at attosecond andfemtosecond lifetimes. One experimental report indicates that therecovery to the ground-state of the electronic structure after acore-level ionization may take up to nanoseconds49 which isconsiderably longer than the XFEL pulse length and wouldexclude further core-level ionization events because of theincreased ionization threshold. Finally, different recovery timesto the ground state, varying between the 1s core-level vacancylifetime of 0.55 fs up to 1 ns, were assumed in the calculations. Inthe simplest approximation, a Gaussian envelope was used todescribe the temporal structure of the incident pulses. Thecalculated dependences are plotted in Fig. 4c, d for TPA and SA,respectively. The same dependencies on the recovery time can beobserved.
Using the shortest possible lifetime of the final state with avacancy in the 1s shell, both the TPA and the SA data cannot bereproduced. The stochastic distribution in the temporal domainwas included by assuming a spiky-structure on the top of theGaussian envelope for the TPA rates. The temporal incident pulsestructure was generated using a coherence time of 0.5 fs inagreement with the expected LCLS machine parameters50.Examples of the incident pulse traces used in the simulationsare shown together with the calculated dependence for thesecond-order coherence factor in Supplementary Note 2.Compared to the smooth Gaussian envelope in the time domainan increase in the calculated TPA rates by a factor of 1.9 can beobserved but still, the experimental data is largely underestimated.Indeed, the experimental data for incident intensity dependencesof TPA and SA can only be reproduced when considering alifetime of the final state which is longer than the duration of theincident SASE pulses. The time-dependent rate equations revealthat the time needed to recover the ground state electronicconfiguration has to be longer than 50–100 fs. Further, thesimulations show that the TPA dependence on the incidentintensity can indeed be described using a relatively simple three-level atomic model which was based on the assumed sequentialexcitation process.
DiscussionWhen exposing atoms to intense X-ray beams with photonenergies varying around a core-level ionization threshold a two-fold behavior is observed in terms of increased transparencyabove the ionization threshold accompanied by an increase inopacity below the ionization threshold. The simultaneous reve-lation of this inverse behavior illustrates how spectroscopy atXFELs can contribute to investigations of intense X-ray interac-tion with matter. Since RXES allows for an element-sensitivedisentanglement of the excitation and decay paths of the atomicstates, nonlinear X-ray processes, such as sequential ionizationand TPA, can be explored in more detail. For example, thechanges observed around the ionization threshold were in arecent report based on core-hole model calculations attributed tothe high-density excitation of 1s electrons which in turn lead to amodification of the energy levels of the remaining, unexcited 1selectrons17. The two-dimensional X-ray spectroscopy resultsreveal, however, two nonlinear ionization channels in the vicinityof an absorption threshold with increasing incident X-rayintensity. The SA mechanism induces a decrease in absorbanceabove the threshold while the TPA process is connected to higherabsorption rates below it. As shown in Fig. 4, the presence of bothprocesses in the vicinity of the ionization threshold causes theobserved absorption edge blurring. In the context of TPAexperiments36,51,52 and X-ray/optical frequency summing, thereconstruction methodology can contribute to a more accurateinvestigation of the X-ray intensity dependence of the inter-mediate virtual states on the recorded TPA yields for differentincident photon energies around the ionization threshold. Con-sidering the time scales of electronic rearrangement processes, aunique advantage of using the reconstruction methodology con-sists in preserving the pulse duration. Indeed, the shortest avail-able pulses are presently on the single femtosecond time scale.Sub-femtosecond pulses, which in have thus a simliar dura-tion than the processes in the electronic structure followingionization events, will be available in future and will enableoriginal studies.
Furthermore, the efficiency of the presented reconstructionmethodology makes it conceivable to record RXES maps withintotal acquisition times ranging from 10–3 s to 10 s, dependingsolely on XFEL facility repetition rate and detector capabilities.
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This opens alleys for transient ultrafast (transient) RXESexperiments at XFEL facilities capable of following completeelectronic rearrangement processes in real time, including laser-pump X-ray probe and X-ray pump X-ray probe experiments53.In particular, quadrupole transitions can be studied, provided thatthe low intensity features can be reconstructed in details, duringcatalytic processes involving transition metals to gain access tothe 3d electronic states. These states are directly involved inmolecular bond formation and dissociation and thus underpinenzymatic processes respectively account for numerous catalyticsystems being developed.
MethodsXPP instrument. The experiments were conducted at the X-ray Pump–Probe(XPP) instrument54 of the Linac Coherent Light Source (LCLS)3. The number ofphotons per pulse and the photon pulse energy were derived from the energy lossof the electron beam and corresponded to 1.29 × 1012, and 1.45 mJ. The repetitionrate of the machine was 120 Hz, the average X-ray pulse duration was ~35 fs(FWHM), and the mean photon energy was varied around 7123 eV by ramping theelectron beam energy up and down at a 5 Hz frequency. The manipulation of theelectron beam energy created a varying incident photon energy distribution for aseries of SASE pulses on a broad enough range to be of interest for RXESexperiments. The mean spectrum had a FWHM of 29 eV. The photon energybandwidth of each single XFEL pulse was about 14 eV FWHM (0.2%). The shortpulse duration was achieved by running the LCLS machine in the low charge mode.
For intensity monitoring purposes, scattering signals along the beam path weresurveyed with diodes and used to ensure a linear response between the diodesthemselves and concerning the intensity registered by a gas monitor and by thetransmission spectrometer.
Sample. The sample was a 4% wt. aqueous dispersion of iron (III) oxide nano-particles having an average size of 30 nm to which 5 mL polypropylene glycol anti-foaming agent was added. The sample was delivered through a sapphire jet nozzleattached to a peristaltic pump drive and head which allowed for a reliablereplenishing of the sample between successive XFEL pulses. The sapphire nozzlehad a width of 200 μm along the beam axis. The liquid jet delivery system wasmounted on a goniometer stage and adjusted such that the XFEL pulses wereincident within the laminar flow regime.
Transmissive spectrometer. The spectral distribution of each incident SASEpulse was monitored in a non-invasive manner, with high-energy resolution and ina single-shot approach with a thin bent crystal transmissive spectrometer25. Thespectrometer was operated in vertical reflection geometry to minimize the impactfrom the uneven beam profile. A Si (400) crystal with a bending radius of 50 mmand a thickness of 10 μm was used to monitor a nominal energy range from 7090eV to 7170 eV. This energy range is broad enough to cover the full bandwidth ofeach XFEL pulse, accounting also for the jitter in electron energy by the Verniermode, and is more than sufficient for the incident photon energy range in theRXES maps. The central photon energy of 7130 eV had a Bragg angle of 39.822°and the nominal photon transmission of the XFEL pulses was 77%. The part of thebeam which is elastically scattered was recorded with an ORCA detector. Thecamera has 2048 × 2048 square pixels with a side length of 6.5 μm, and is capable ofa 120 Hz frame rate. It is coupled to a 30 μm YAG:Ce screen and positioned attwice the Bragg angle concerning the XFEL pulse axis. The distance separating thecrystal and the camera was 250 mm. This arrangement resulted in a nominalenergy range of 52 meV per pixel along the dispersive direction, which is sufficientto resolve the single spikes in the SASE spectra when considering the extremelysmall angular divergence of the XFEL beam.
Downstream of the sample, at a distance of about 2 meters, an identical thinbent crystal transmissive spectrometer was operated to monitor the photonstransmitted through the sample. For this spectrometer a Si (220) crystal bent to aradius of 50 mm was operated at a Bragg angle of 27°.
The energy axis of the transmissive spectrometer was calibrated in conjunctionwith the downstream transmission spectrometer by using the large offsetmonochromator of the XPP instrument55 for the latter spectrometer and bymatching the central position and standard deviation of the mean spectra acquiredwith both spectrometers during the energy scan. No sample and no focusing opticswere inserted in the beam path during this measurement. A Fe foil upstream of thetransmissive spectrometer was used to calibrate the absolute energy axis of thespectrometers. The response function of the transmissive spectrometer wasestablished by recording at a fixed spectrometer setting the average SASE spectra atdifferent XFEL beam energies, normalizing the spectra to the intensity registered byan upstream gas monitor and fitting a polynomial function through the peakpositions of the individual mean spectra. Moreover, dark image and backgroundcorrections were applied to each recorded image.
For each measurement at a given intensity, it was imposed that only spectrawithin an intensity window of 2 times the standard deviation of the recorded
integral intensity were considered when reconstructing the RXES maps. In additiononly those spectra were considered whose center of mass in the dispersive and thenon-dispersive direction, respectively, were within a window having a width of 2times the standard deviation of the measured individual centers of mass: in thenon-dispersive direction this allowed to preserve a constant source position for thevon Hamos spectrometers, whereas in the dispersive direction this allowedrejecting SASE pulses where possibly a part of the spectrum was outside themonitored detector region.
X-ray emission spectrometer. The Fe Kα and Kβ XES signals were each mon-itored on a single-shot basis in high-energy resolution using two separate crystalspectrometers operated in the von Hamos geometry, allowing for a scanning-freeacquisition of the XES data within a relatively larger energy band26. Both spec-trometers used cylindrically curved crystals with a radius of curvature of 250 mmmounted in a horizontal geometry to minimize the scattering background. Thedispersion axis was perpendicular to the axis of the incident pulses. The crystaldimension in the dispersive and focusing directions was 50 mm and 100 mm,respectively. For the Kα line a Ge (440) crystal was used, while for the Kβ and V2Cline an InSb (444) crystal was selected. The respective Bragg angles for the Kα1 andKβ1,3 lines were 74.47° and 69.93°. Hence the distance along the dispersive axisfrom the crystal center to the beam axis was 64.8 mm and 91.3 mm. In bothspectrometers, a position-sensitive CSPAD-140k detector (388 × 370 pixels – dis-persive and non-dispersive direction, 110 × 110 μm2 pixel size, 120 Hz readoutrate)56 was installed to monitor the XES signals. The detector surface contained thedispersion axis and was normal to the axis of the incident XFEL pulses. Leadshielding was used to prevent any scattering of the XFEL pulses from air or watertowards the detector. For the Kα and Kβ emission lines, this resulted in an averageenergy bandwidth of 0.34 eV per pixel and 0.5 eV per pixel, respectively. Theexpected energy resolution for the described configurations was 0.5 eV for thespectrometer monitoring the Kα emission line and 0.7 eV for the one monitoringthe Kβ emission line.
To extract the XES data from the CSPAD-140k detector, a region of interestalong the focusing direction was defined before projecting the data on thedispersive axis. Before the projection, a dark image correction, a backgroundcorrection and a pedestal were applied and permanent and temporary pixel defectswere corrected for. The energy scale of the XES spectra was calibrated using the Kαspectrum recorded at the lowest intensity together with the nominal values for FeKα1 and Kα2 (6403.84 eV and 6390.84 eV) whereas for the Kβ signal the envelopeof the elastic signal from the aqueous sample was considered to establish an energyscale which is congruent with the transmissive spectrometer using a measurementwithout focusing optics in the incident beam. The stability of the XES sourceposition, which impacts directly the XES energy axis, was confirmed by the use oftwo von Hamos spectrometers installed on either side of the XFEL pulse axis asshown in Fig. 1 and Supplementary Figure 4.
The recorded spectra were normalized to the identical incident intensitymonitoring signal registered by an air scattering diode, just like the spectrarecorded by the transmissive spectrometer. For each measurement at a givenintensity, it was considered that the intensity ratio registered between thespectrometers for the Kα respectively the Kβ XES signal should be constant withina tolerance of 3 times the standard deviation of the ratio measured for theindividual pulses. Following the filtering of the data from the transmission and thevon Hamos spectrometers, between 23,893 and 68,653 pulses, representing onaverage 86% of the initially recorded pulses, were used per incident X-ray intensityfor the reconstruction.
Reconstruction of the RXES maps. At synchrotron radiation facilities RXES mapsare measured using monochromatized radiation and by recording at each incidentphoton energy Ei, i varying from 1 to m, an X-ray emission spectrum Si composedof n elements. Upon normalizing each incident spectrum to the incident intensityIi, the RXES map can be described as a matrix Rmn, where each row corresponds tothe normalized emission spectrum: Rmn= Jmm
−1 ∙ Smn with Jmm being a diagonalmatrix containing for each incident energy Ei the incident photon intensity Ii onthe diagonal element and Smn being a matrix in which each row corresponds to therecorded spectrum Si for the incident photon energy Ei.
At an XFEL operated in the SASE regime, the matrix Jkm is no longer diagonalsince each incident photon pulse is described as a vector of m components withnon-zero intensities at different incident energy positions nor is Jkm square sincethe number of recorded pulses k can be different from the number of incidentenergy components m. Due to the stochastic XFEL pulse generation process, eachincident energy spectrum Jj, j varying from 1 to k and k being larger than m whenacquiring a sufficient number of SASE pulses, is linearly independent from all otherincident energy spectra. Hence the matrix has a full column rank and a left inversecan be defined as (JkmT ∙ Jkm)−1 ∙ JkmT which is equal to the Moore-Penrosepseudoinverse matrix Jkm+. The X-ray emission spectrum Sj corresponding to thepulse with spectrum Jj can be described as a linear combination of the X-rayemission spectra expected for monochromatized radiation, thus the X-ray emissionspectra collected over a series of k SASE pulses corresponds to Skn= Jkm ∙ Rmn. TheRXES map is reconstructed from the measurement of the spectra of the incidentSASE pulses with photon energies varying around an elemental ionizationthreshold and of the appertaining X-ray emission spectra using Rmn= Jkm+ ∙ Skn.
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The simplest case for the reconstruction is considered in Supplementary Fig. 5. Theenergy resolution is in a first approximation limited only by the energy resolutionwith which the spectra of the incident photons and of the emitted photons wererecorded. From the qualitative comparison of the RXES maps reconstructed for thelowest incident intensity to synchrotron data, as it is done in Supplementary Fig. 1,it is inferred that the energy resolution of the reconstruction methodology is ofabout 1 eV on the incident energy axis and better than 1 eV on the emissionenergy axis.
Data availabilityRaw data were generated at the X-ray Pump Probe instrument of the Linac CoherentLight Source. Derived data supporting the findings of the presented study are availablefrom the corresponding authors on reasonable request.
Received: 20 February 2019; Accepted: 25 September 2019;
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AcknowledgementsUse of the Linac Coherent Light Source (LCLS), SLAC National Accelerator Laboratory,is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy
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Sciences under Contract No. DE-AC02–76SF00515. We acknowledge the Paul ScherrerInstitut, Villigen, Switzerland for provision of synchrotron radiation beamtime at theX05LA-MicroXAS beamline of the SLS. M.L. acknowledges a grant from the Knut andAlice Wallenberg Foundation (Grant No. KAW-2013.0020). J.Sz. acknowledges partialsupport by National Science Centre (Poland) under grant no. 2015/18/E/ST3/00444 and2015/19/B/ST2/00931. W.B and J.Sá acknowledge partial support of the Polish Ministryof Science and Higher Education from the budget allocations for science for years2016–2019 within the IDEAS PLUS II project of number IdPII2015000164.
Author contributionY.K. and J.Sz. developed the idea and conceived the methodology for the reconstructionof the RXES maps and elaborated in conjunction with C.M., P.J., D.Z., and R.A. theexperimental concept for the XFEL experiment. Y.K., J.Sz., C.M., and J.C.-M. performedthe synchrotron comparative experiments. Y.K., J.Sz., C.M., P.J., J.Sá, L.S., D.Z., and R.A.M. prepared and coordinated the experiment (setup of XES spectrometer, sampleenvironment, respectively transmission spectrometer and beamline setup) and in colla-boration with J.C.-M., R.F., M.K., G.K., and J.R. carried out the experiment. Y.K. per-formed with the help of J.Sz., C.M., P.J., W.B. and L.S. the analysis of the experimentaldata. K.T and J. Sz. performed time-dependent rate equation calculations, W.B. madeMonte-Carlo simulations on transfers of vacancies in the electronic configuration and M.D. and M.L. did XATOM and multiplet calculations. Y.K., J.Sz., J.Sá, C.M., and P.J.interpreted the results and Y. K. and J.Sz. wrote the manuscript with input and remarksfrom all authors.
Competing interestsThe authors declare no competing interests.
Additional informationSupplementary information is available for this paper at https://doi.org/10.1038/s41467-019-12717-1.
Correspondence and requests for materials should be addressed to Y.K., J.S. or J.S.
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ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-12717-1
10 NATURE COMMUNICATIONS | (2019) 10:4761 | https://doi.org/10.1038/s41467-019-12717-1 | www.nature.com/naturecommunications