Soft X-ray Absorption Spectroscopy of Aqueous Solutions Using a

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Spectroscopy and Photochemistry; General Theory

Soft X-ray Absorption Spectroscopy of Aqueous Solutions Using a Table-Top Femtosecond Soft X-ray Source Carlo Kleine, Maria Ekimova, Gildas Goldsztejn, Sebastian Raabe, Christian Strüber, Jan Ludwig, Suresh Yarlagadda, Stefan Eisebitt, Marc J.J. Vrakking, Thomas Elsaesser, Erik T. J. Nibbering, and Arnaud Rouzee J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03420 • Publication Date (Web): 14 Dec 2018 Downloaded from http://pubs.acs.org on December 14, 2018

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The Journal of Physical Chemistry Letters

Soft X-ray Absorption Spectroscopy of Aqueous Solutions Using a Table-Top Femtosecond Soft X-ray Source Carlo Kleine, Maria Ekimova, Gildas Goldsztejn, Sebastian Raabe, Christian Strüber, Jan Ludwig, Suresh Yarlagadda, Stefan Eisebitt, Marc J. J. Vrakking, Thomas Elsaesser, Erik T. J. Nibbering,∗ and Arnaud Rouzée∗ Max-Born-Institut f¨ ur Nichtlineare Optik und Kurzzeitspektroskopie, Max-Born-Strasse 2a, 12489 Berlin, Germany E-mail: [email protected]; [email protected]

Abstract

Graphical TOC Entry

We demonstrate the feasibility of soft X-ray absorption spectroscopy in the water window using a table-top laser-based approach with organic molecules and inorganic salts in aqueous solution. A high-order harmonic source delivers femtosecond pulses of short wavelength radiation in the photon energy range from 220 eV to 450 eV. We report static soft X-ray absorption measurements in transmission on the solvated compounds CO(NH2 )2 , CaCl2 , and NaNO3 , using flatjet technology. We monitor the absorption of the molecular samples between the carbon (≈ 280 eV) and nitrogen (≈ 400 eV) Kedges and compare our results with previous measurements performed at the BESSYII facility. We discuss the roles of pulse stability and photon flux in the outcome of our experiments. Our work paves the way towards table-top femtosecond, solution phase soft X-ray absorption spectroscopy in the water window.

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X-ray absorption spectroscopy (XAS) is an element-specific tool to locally probe the chemical environment of a particular atom. 1 XAS has intensively been used in the energy range between 100 eV and 1000 eV, as the main chemical constituents of many organic and biological compounds, C, N and O, have their absorption K-edges in this soft X-ray spectral range. 2 In particular, probing unoccupied orbitals through 1s excitations at the N and O K-edges has provided profound insight into the electronic and structural aspects of hydrogen bonding and underlying charge distribution effects in both steady-state 3–7 and time-resolved experiments. 8–10 In addition, the L-edges of 3d metal ions located in the same spectral range and above the O K-edge has provided direct access to the frontier orbitals of a large number of important metal-ligand complexes. 11,12 Up to now XAS in the soft X-ray spectral range has successfully been pursued at large scale facilities, such as storage rings and X-ray free electron lasers, 2,13–16 and using laboratoryscale, table-top, plasma-based light sources. 17 At large scale facilities, beamlines offer wellfocussed, strongly directed beams with high photon fluxes at sufficient spectral resolution, but the limited availability of beamtimes at these highly competitive user facilities, especially when femtosecond X-ray pulses are required, limits the progress that can be achieved. Laboratory-scale, plasma-based sources generally do not face beamtime restrictions, but the incoherent nature of the X-ray emission demands special care when directing the X-ray source with suitable X-ray optics towards the sample target and subsequent spectrally resolving detectors. Moreover, it is challenging to create plasma conditions where the desired broadband soft X-ray emission is produced in femtosecond pulses permitting high temporal resolution studies. Alternatively, recent developments in high-order harmonic generation (HHG) 18–22 have demonstrated the potential of these ultrafast laser-based pulsed soft X-ray sources as a means to steady-state 23 and timeresolved soft X-ray spectroscopy well into the water window and beyond. 24–26 Time-resolved XAS based on the use of an HHG source has

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already been applied to bond-breaking and rearrangements of molecular systems in the gas phase occuring on a femtosecond timescale. 24,25 To expand the potential of table-top HHG sources towards the solution phase, where a major part of chemistry takes place, the integration of these laser-based soft X-ray sources with liquid jet technology is necessary. Liquid jets have been applied in soft X-ray spectroscopic measurements at synchrotron facilities for almost two decades, 2,27,28 particularly using photoelectron and Auger, X-ray emission (XES) and resonant inelastic X-ray scattering (RIXS) spectroscopic methods. The approximately 20 µm diameter of single round jets has prevented the application of XAS measurements in transmission. This problem has recently been overcome with the development of liquid flatjets. 29–31 Using two colliding liquid jets, a stable flatjet has been demonstrated as a tool for XAS in transmission mode. Up to now, the flatjet technique has been applied in N K-edge absorption spectroscopy for hydrogen bond research of amine compounds, 7,32 as well as in L-edge spectroscopy of charge and spin densities of 3d metal ions complexed with nitrogen- or oxygen-containing ligands. 33,34 In this paper, we demonstrate the successful combination of the liquid flatjet approach with a laser-based HHG set-up, and report measurements of soft X-ray absorption spectra of several solutes in aqueous solution. The analysis of our experimental results in term of the photon count rate and the pulse stability enables us to provide the roadmap towards highresolution, time-resolved soft X-ray absorption spectroscopy of solutes in solution with femtosecond resolution. High-order harmonic emission is a highly nonlinear process 35,36 in which a femtosecond laser pulse is used to ionize a gas sample. Due to the rapidly oscillating laser field, the ejected electrons are accelerated by the laser field before being driven back to their parent ions. Recombination processes are accompanied by the emission of high energy photons. Due to the periodic nature of the laser field, ionization, acceleration and recombination take place every half-cycle of the laser field, resulting in the


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Liquid flatjet

Differential pumping stage

5 mm gas cell 1.8 mm 60 fs, 2 mJ

CaF2 lens 30 cm

Ni coated toroidal mirror

10-4 mbar 10 mbar 100 nm thin Al/Ti filter

VLS grating

10-6 mbar Catcher 10-3 mbar

Differential pumping stage

Figure 1: Sketch of the experimental set-up. In order to generate the high harmonics, a 1.8 µm laser pulse is focused inside a 5 mm long gas cell filled with ≈ 4 bar of helium. The soft X-ray pulse is separated from the remaining short wavelength infrared (SWIR) pulse using a 100 nm thick Al or Ti filter and is focused onto the liquid flatjet by a toroidal mirror. The absorption is monitored using a flat field spectrometer composed of a variable line spacing (VLS) grating and an MCP/phosphor screen assembly. emission of an attosecond pulse train with a frequency spectrum composed of odd harmonics of the laser field. The maximum photon energy in the process is given by the well-known microscopic single-atom cutoff law E = IP + 3.17Up , with IP the ionization potential of the gas and Up [eV ] = 9.33 · I[1014 W/cm2 ] · λ2 [µm] the ponderomotive energy of the laser field. The maximum photon energy in HHG can be tuned by increasing the laser wavelength or the laser intensity. In doing so, macroscopic phase-matching conditions limit the maximum intensity that can be used. At high pulse intensity, ionization of the gas sample becomes substantial and plasma dispersion creates a mismatch between the phase of the driving laser and the harmonic fields. 37 Based on the wavelength scaling of Up , significant scaling of the photon energy can be achieved in HHG by increasing the driver laser wavelength. However, the high harmonic efficiency drops dramatically with increasing laser wavelength, with a dependence given by λ−5 to λ−6 . 38 To overcome this limitation, efficient phase-matched HHG

schemes above the carbon K-edge have been implemented using extended high density media such as gas-filled hollow waveguides 19,20,37 and differentially pumped gas-cell systems. 22,39 In the former case, a non-diverging plane-wave phase-matching condition is achieved over an extended distance inside the gas-filled capillary, thereby overcoming the geometric phasemismatch induced by focusing the driver laser pulse. The large number of coherently emitting atoms in the gas-filled capillary then allows for generating bright coherent X-ray beams even at moderate laser intensities. 22 In the latter case, efficient high harmonic emission is achieved in a gas cell by strongly ionizing the generation medium. The plasma resulting from ionization at the entrance of the gas cell is responsible for a spatiotemporal reshaping and a subsequent self-guiding of the driver laser due to the competition between plasma defocusing, geometrical focusing and self-focusing. 22,39 Eventually, the intensity of the driver laser is clamped as it propagates in the gas cell, improving the phase-matching over a long distance and en-


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running the liquid flatjet. A typical HHG spectrum measured over 7 · 105 laser shots is shown in Fig. 2 a) and b). An optimum yield was achieved for a helium pressure of 3.8 bar and a pulse energy of 1.6 mJ (peak intensity of 4.7·1014 W/cm2 ). In this configuration, we detected a total flux across the measured spectrum spanning photon energies from 280 to 450 eV of (3.5 ± 0.5) x 103 photons · s−1 (at a repetition rate of 360 Hz defined by the camera settings). Taking into account a 10 % average detection efficiency of the MCP detector in the soft X-ray spectral range, 40,41 the 2% transmission efficiency of the VLS grating previously characterized using a soft X-ray plasma source and a 66% reflectivity expected for the Ni-coated toroidal mirror, we estimated a minimum total flux at the source of (7.4 ± 1.0) x 106 photons · s−1 and a pulse energy of 0.35±0.05 pJ. These estimated values are slightly lower than recently reported HHG fluxes in a He-filled gas cell performed in similar conditions but using either a higher pulse energy 39 or few-cycle, 1.8 µm driver laser pulses. 22,42 We note as well that the pulse energy achieved in our experimental conditions is three order of magnitude lower than the HHG reported in gas-filled hollow waveguides. 19 The measured high harmonic spectrum extends to a maximum photon energy of 440 eV, i.e. slightly lower than previous investigations. 22,39,42 Harmonic lines spaced by twice the laser frequency (1.38 eV) are clearly distinguishable up to a photon energy of 320 eV, beyond which the spectrometer resolution becomes too low to separate them. Such clear harmonics have not been observed in previously reported measurements. 22,39,42 This is likely due to the lower peak intensity and longer pulse duration used in our experiment. By fitting the harmonic lines using a convolution between two Gaussian functions to represent the instrumental resolution and the natural harmonic bandwidth (estimated below 150 meV for an attosecond pulse train with a 30 fs pulse envelope, FWHM), an accurate determination of the spectrometer resolution is possible. We found E/∆E ≈ 220, which corresponds to an energy resolution of ∆E ≈ 1.2 eV near the C K-edge

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Counts/30 shots

hancing the high-harmonic emission at high photon energies. When employed with a multimJ, 1.8 µm laser pulse, a bright soft X-ray beam with 2.9·103 photons/shot/1% bandwidth around the carbon K-edge (280 eV) has been recently achieved. 39 A similar scheme has been implemented in our study and is shown in Fig. 1. Detailed information on the experimental set-up are presented in the Experimental Methods section.

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Figure 2: a) CCD image of high harmonics recorded with the soft X-ray spectrometer and corresponding spectrum b) obtained by vertical integration of the image and calibration of the detector. In the inset, we show an expanded view of the harmonic spectrum below 300 eV. The dip observed in the spectrum near 280 eV is due to contamination of the X-ray optics by carbon-containing compounds. c) Time evolution of the total photon count rate averaged over 30 laser shots (black dots) and averaged over 1 minutes (red line). d) Time-evolution of the ratio between the standard deviation ∆n and the q square root of the average photon count rate hni. e) Power spectrum of the total soft X-ray photon flux obtained by Fourier analysis. The power spectrum for a Poisson distribution is shown for comparison To estimate the available soft X-ray flux, high harmonic spectra were first recorded without


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and ∆E ≈ 1.9 eV near the N K-edge.

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characteristics q are close to Poissonian at early times (∆n/ hni ≈ 1), they evolve to stable conditions on a timescale of 1 hour which the ratio almost doubles. We ascribe this behavior to drift of the laser parameters, such as the beam pointing that was not actively stabilized. A closer inspection of the power spectrum of the total photon flux (see Fig. 2 e)) shows that whereas a shot-noise limited source would lead to a power spectrum that is flat over a frequency range fromm 0.001-10 Hz, the laser source departs from Poisson-like behavior in the frequency range below 1 Hz, corresponding to temporal fluctuations occurring on timescales longer than 1 s. The same analysis performed for the photon flux measured within a 1 % bandwidth at the C and N K-edges (see Fig. 3) reveals that the measured flux at these edges is shot-noise limited on a timescale below 1 minute. To minimize the effects of long term drifts of the photon flux near the two Kedges, we can therefore perform series of measurements with and without the flatjet using an integration time lower than 1 minute. Soft X-ray absorption spectra recorded using our table-top HHG set-up obtained for aqueous solutions of 1 M urea (O=C(NH2 )2 ), 0.5 M calcium chloride (CaCl2 ) and 0.75 M sodium nitrate (NaNO3 ) are shown in Fig. 4 (thick black solid lines). The results have been obtained by averaging a minimum of 20 measurements using Eqs. 1 and 2 given in the Experimental Methods section, with each measurement averaged over 1 minute to prevent the additional temporal fluctuations occurring on longer timescales. We also recorded reference spectra of the solvent water, shown as dashed lines, using an average over 15 different measurements. We note that all spectra were smoothed using a 5 pixel running boxcar average on the CCD image. The grey-shaded curves indicate the error margins of the XAS measurements, estimated by Eqs. 2 (see Experimental Methods section). For further comparison, we also display in panels 4 a) and e) XAS measurements near the C K-edge (blue solid line) and the N K-edge (orange solid lines) of urea and NaNO3 aqueous solutions, as recorded at the UE52 SGM beamline at the synchrotron radiation source BESSY II

HHG (1% bandwidth C K−edge) Frequency of XAS measurements

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Figure 3: Power spectrum of the photon flux measured within a 1 % bandwidth at the C K-edge a) and N K-edge b) (black lines) compared to the power spectrum expected from a shot-noise limited source that satisfies a Poisson distribution (blue lines). The dashed red lines corresponds to the flatjet on and off frequency used to measure the absorbance of all tested samples.

Since high harmonic generation is a highly nonlinear process, it is susceptible to small changes of the driving laser parameters and it is therefore important to quantify the stability of the soft X-ray photon source. The fluctuations are characterized by monitoring the total number of detected photons n within 30 laser shots (corresponding to 1 camera frame) during a period of 100 minutes (see Fig. 2 c)). Overall, the photon count rate fluctuates with a standard deviation of 14 % around an average value hni ≈ 295. We also observe a slight decrease of the photon flux over time. A shotnoise limited detection should satisfy a Poisson distribution with a standard deviation ∆n that q would be given by hni. The ratio between these two quantities averaged over 1 minute intervals in the case of our soft X-ray source is displayed in Fig. 2 d). While the source


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The Journal of Physical Chemistry Letters 0.4

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XAS measurement measured for aqueous urea at BESSY II near the carbon K-edge is composed of a sharp 1s-π ∗ transition at 289.5 eV, a shoulder at 294.1 eV and a broad resonance corresponding to the 1s-σ ∗ transition centred at 300 eV (FWHM ≈ 8 eV). The nitrogen K-edge spectrum of urea is dominated by a relatively strong pre-edge feature at 402.6 eV, followed by a main-edge transition located at 403.7 eV, and two post-edge peaks that can be observed at 408.6 and 412.2 eV. These values are in line with previously reported electron-energy loss spectra on solid urea, 43 and partial fluorescence yield XAS measurements of aqueous urea. 44 For aqueous NaNO3 we observe a narrow 1sπ ∗ transition at 405.0 eV at the N K-edge, and a broad 1s-σ ∗ transition centred at 415 eV (FWHM ≈ 10 eV), in agreement with previously reported data from a total electron yield XAS study on aqueous NaNO3 . 45 Additionally, the absorption spectrum of CaCl2 is composed of the L3 ,L2 -transitions at 350.5 eV and 353.6 eV, with relative absorption strengths in good agreement with results reported on solid CaCl2 .H2 O. 46 In order to compare the HHG-based and BESSY synchrotron-based measurements, we also show an expanded view of the spectra near the C K-edges, N K-edges and Ca L3,2 edges of the three samples in Fig. 4 b),d) and f). Here we have substracted the solvent contributions for clarity purposes. Moreover, we have convoluted the measurements obtained at the UE52 SGM beamline at BESSY II with a Gaussian function with a FWHM of 1.2 eV at the C K-edge and 1.9 eV at the N K-edge to match the spectral resolution (E/∆E ≈ 220) of the spectrometer used in the table-top HHG set-up. The XAS results obtained with the table-top HHG set-up and the spectrally broadened XAS results measured at the UE52 SGM beamline agree within the error bars of the former measurement for the C K-edge spectra of aqueous urea. Despite the 7 orders of magnitude lower photon flux at the N K-edge spectral region provided by the table-top HHG set-up compared to the synchrotron measurement, the results for urea and NO− 3 also match the synchrotron results within the error bars. Hence we conclude

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O=C(NH2)2

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Figure 4: In panels a), c) and e) the absorbance of O=C(NH2 )2 , CaCl2 and NaNO3 (black solid lines) recorded with our table-top XAS set-up is shown together with their corresponding confidence intervals (grey areas). The absorbance of the pure water solvent is displayed as well (dashed lines). We note here that since the absorbance is a function of the flatjet thickness, it is important to independently characterize the water solvent absorption for all samples. The XAS measurements obtained at the UE52 SGM beamline at BESSY II near the C-K-edge (blue) and N-K-edge (orange) are shown in panels a) and e). Panels b), d) and f) show a zoom near the C and N K-edge for the three samples, corrected for the absorption from the pure solvent. Here, the spectra taken at BESSY II (blue dashed and orange dashed lines) have been convoluted with a spectral broadening function to match the resolution of the table-top HHG setup. at the Helmholtz-Zentrum Berlin, for the same concentrations of the two samples. We note that the experiments at BESSY II have been performed using the same procedure that was used in our previously reported measurements on amine compounds 32 and the spectra shown in Fig. 4 have been corrected for the absorption by the solvent. In agreement with the BESSY II results, the


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that it is possible to probe spectral structure at the C and N K-edges of organic and inorganic solutes at (sub)-molar concentrations using our table-top soft X-ray source. We note that the S/N ratio achieved at the N K-edge of sodium nitrate was lower with respect to the other tested samples due to a decrease of the photon flux observed near the cutoff energy of the harmonics spectrum during this run. The low photon flux recorded is responsible for a poorer S/N ratio and the appearance of artifacts (additional peaks above 410 eV in the Fig. 4). By increasing the number of photons above 410 eV, we expect to be able to reach a S/N ratio identical to the one measured at the C Kedge in urea. Compared to a synchrotron experiment, one of the advantages of our table-top HHG approach for steady-state soft X-ray spectroscopy in the water window clearly lies in the simultaneous probing of the full spectral range from 250 to 430 eV without relying on scanning the detected photon energy. Any variation of flatjet thickness during the data accumulation can easily be characterized by inspection of the spectral responses just below the atomic K-edges. In contrast, a correction for flatjet thickness variation is not a trivial procedure when working with scanning monochromator devices at storage ring beamlines, since recording reliable spectra necessitates a proper calibration of the sample conditions during the full measurement times. Another advantage in directly probing the full spectral range lies in the potential of a direct comparison of the molecular response at e.g. the C and N K-edges, in particular when experiments of sample series are pursued with systematic changes of molecular functionalities. Here one can consider measuring a series of molecular compounds with a systematic change in substituents, to explore particular molecular design routes. In addition, the effects on the C and N K-edges of molecular systems undergoing slow structural changes can also be more directly correlated. Here one could think of polymer phase transformations or protein folding of biomolecular systems, e.g. triggered by a temperature-jump or a pH -jump. The real promise of HHG-based XAS lies in

the exploitation of the inherent temporal resolution offered by a table-top HHG set-up. This allows to investigate primary events in physics and chemistry using femtosecond and attosecond pump-probe schemes. Storage ring beamlines have a significantly higher photon flux, on the order of 1012 photons/s/1% bandwidth than our current table-top HHG approach at the expense of a rather large pulse duration on the order of 50 ps. Pulse durations reaching the femtosecond timescale are available with slicing facilities but with photon fluxes comparable to those of laser-based HHG soft X-ray setups (typically 106 photons/s/1% bandwidth 47 ). Therefore, the HHG approach enables the systematic investigation of ultrafast chemical dynamics of a larger set of molecular systems and functionalities and even more subtle molecular rearrangements, without the restriction of the limited beamtime. We note that a similar restriction applies at free electron lasers, even if the fluxes available at these facilities are by far higher than the one achieved with a laser-based approach. 48 The current S/N ratio achieved in our experiment is compromised by the fact that only a small fraction of the generated photons are detected, as a result of the finite efficiency of both our X-ray optics and our X-ray detectors. Hence, significant gains are possible by improving these optics and detection systems, for instance using high repetition rate X-ray detectors and high reflectivity gratings based on zone plate technology. 49 An active stabilization of the driver laser will also lead to a large improvement of the S/N ratio and will allo shotnoise limited operation over longer time intervals. With this we are confident that a full development of femtosecond UV-vis-pump soft Xray-probe spectroscopy of molecules in solution is possible. In conclusion, we have shown the possibility to perform soft X-ray absorption spectroscopy in the water window spectral range on aqueous solutions in transmission, using a table-top high harmonic source and a liquid flatjet. Our results are in good agreement with measurements performed using synchrotron radiation with 7 to 8 orders of magnitude higher photon flux. Further improvements of the HHG


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source in soft X-ray spectral range extending towards 1 keV will increase the potential of the table-top HHG method, for instance for probing the frontier orbitals of 3d-metal-ion-complexes through L-edge transitions. 33 Our results using a table-top femtosecond HHG source utilizing flatjet technology pave the way for both steadystate and ultrafast soft X-ray spectroscopy of solution phase molecular systems. Ultimately probing elementary transformations of chemical bonds through the frontier orbitals with laser-based HHG soft X-ray absorption spectroscopy will lead to key new insights into charge flow, proton transport and bond rearrangements in photoinduced chemistry in the condensed phase.

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to remove the SWIR beam from the generated soft X-ray beam. A Ni-coated toroidal mirror with 4◦ grazing incidence angle was employed to image (1:1) the SWIR focus inside the HHG chamber onto a liquid flatjet system (Microliquids GmbH, Göttingen). 29 The liquid flatjet has previously been used to perform soft X-ray absorption experiments at the synchrotron radiation source BESSY II at the Helmholtz-Zentrum Berlin. 32 Briefly, in the flatjet set-up, a stable liquid sheet is formed upon collision of two identical laminar jets coming out of two separate nozzles with 50 µm orifices, under an angle of 48◦ . To reach a stable sheet from the two colliding single jets, the solution was directed in the two orifices by a HPLC (high performance liquid chromatography) pump at a flow rate between 5.7 and 6 ml.min−1 . From previous measurements, the sheet was characterized to be 4.6 mm long and 1 mm wide, with a thickness between 1 and 3 µm, depending on the position in the sheet. 29 With the running liquid flatjet, a pressure

Soft X-ray Absorption Spectroscopy of Aqueous Solutions Using a

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