Ionic Current Mn K-Edge X-ray Absorption Spectra Obtained in a Flow

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Ionic Current Mn K-Edge X-ray Absorption Spectra Obtained in a Flow Cell Lifei Xi, Martin Schellenberger, Raphael Francesco Praeg, Ronny Golnak, Götz Schuck, and Kathrin M. Lange J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04693 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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The Journal of Physical Chemistry

Ionic Current Mn K-edge X-ray Absorption Spectra Obtained in a Flow Cell Lifei Xi,† Martin Schellenberger,† Raphael F. Präg,† Ronny Golnak,‡ Götz Schuck,¦ Kathrin M. Lange†,§,* †

Young Investigator Group Operando Characterization of Solar Fuel Materials (EE-NOC),‡ Institute Methods for

Material Development, ¦Department Structure and Dynamics of Energy Materials (EM-ASD), Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, 12489 Berlin, Germany, §Universität Bielefeld, Physikalische Chemie, Universitätsstr. 25, D-33615 Bielefeld, Germany.

ABSTRACT: Ionic current X-ray absorption spectroscopy (IC-XAS) relaying on the synchrotron beam-induced ionization current was recently proposed as an alternative approach for XAS detection in flow cells. In this study, we investigate the mechanism behind IC-XAS by varying the spacer thickness and the cell orientation. Based on these studies, we propose that the driving force for the ionic current is a beam-induced electric potential between the two electrodes. After that, we attempt to detect IC-XA spectra from the Mn K-edge of several manganese salts, specifically nitrate, chloride, sulfate, acetate and permanganate. We find that Mn K-edge IC-XAS spectra of aqueous Mn ions can be affected by the mass and concentration of the anions, as well as the substrate and the beam intensity. Potential applications and limitations of this method are discussed.

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INTRODUCTION As an element-specific probe, X-ray absorption spectroscopy (XAS) provides electronic structure information of a specific absorber atom species contained in solid or solution. XAS at the transition metal K-edge (1s → np transition) is a powerful technique for determining the mean oxidation states of materials. Several detection modes for XAS are well established. The direct approach to measure true X-ray absorption spectra is the transmission mode, where the attenuation of an incoming X-ray through a bulk sample is monitored. For transmission measurements, the thickness of the sample needs to be optimized depending on the sample composition. Indirect methods, eg. the fluorescence yield (FY) or the electron yield (EY) modes, to measure XAS are also widely used. For FY or EY modes core-hole relaxation products (photons or electrons, respectively) are detected upon X-ray absorption. FY measurements are bulk sensitive while EY measurements are usually surface sensitive due to the shorter mean free path of electrons. Ionic current measurements or previously called total ion yield (TIY) measurements1-3,4 are recently emerging as an alternative method for XAS detection in flow cells. Ionic current measurements relay on the current which results from the core-hole relaxation by-products upon Auger electron emission. The characteristic XAS features are obtained when scanning the absorption edges of the investigated elements.4,5 For example, Velasco-Velez et al. studied the structure of water near gold electrodes.5 In that study, they assumed that the signal results from the secondary electrons and claimed that the interfacial water molecules have a different structure from those in the bulk. However, when looking at their circuit diagram it actually seems to be an ionic current detection technique using a two-electrode configuration.6,7 Schön et al. initially obtained O K-edge and Fe L-edge ionic current XA spectra from Fe(NO3)2 aqueous solution using a two-electrode system.4 Recently, they deposited three different self-assembled monolayers (SAMs) of alkane thiols


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(HOOC-(CH2)n-SH, with n= 3, 11, 16) on gold-coated Si3N4 membranes and measured O K-edge spectra of an oxygen-free solvent compared to that with water.7 They proposed that the detected ionic current stems dominantly from the bulk-solution species, while only marginally from the species located at the membrane-solution interface. However, a driving force leading to the ionic current that seems to be proportional to the absorption cross section is not proposed. In addition, effects that could lead to distortions of the IC-XAS spectra with respect to the true absorption measured in transmission mode were not investigated until now. In the present study, we investigate firsts the mechanism behind IC-XAS by varying the spacer thickness and the cell orientation. Based on these studies, we propose a driving force for the ionic current. After that, we attempt to detect IC-XA spectra from the Mn K-edge of several manganese salts: nitrate, chloride, sulfate, acetate and permanganate. We study the influence of the anion type, the concentration of the solution and the substrate as well as the beam intensity on IC-XA spectra and compare them to XA spectra obtained in transmission mode. We also discuss distortions from the spectra obtained in ionic current mode. Finally, we discuss the potential application and limits of this emerging XAS technique. EXPERIMENTAL SECTION Materials. Manganese (II) nitrate hexahydrate (Mn(NO3)2·6H2O, ≥98.0%), manganese (II) chloride tetrahydrate (MnCl2·4H2O, 99.99%), manganese (II) sulfate monohydrate (MnSO4·H2O, Pharmacopoeia Europaea grade), manganese (II) acetate tetrahydrate (Mn(CH3COO)2·4H2O ≥99%), potassium permanganate (KMnO4, 99.0%) are purchased from Sigma-Aldrich. All Mn salt solutions are prepared with DI water (18.6 MΩ·cm). 2 mm x 2mm x 150 nm Si3N4 membranes supported by a Si frame (381 µm) are ordered from Silson Ltd. (UK). Si3N4 membranes are coated with 1-2 nm Ti and 30 nm Au. An Au foil for ohmic contact (127 µm in thickness, 99.99% trace


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metals basis) is ordered from Sigma-Aldrich. Indium tin oxide (130 nm) coated PET (127 µm) sheets (or called PET/ITO sheets) are purchased from Sigma-Aldrich. Characterization. The experiments are conducted at the KMC2 beamline of BESSY II, Berlin, using our recently developed photoelectrochemical transmission flow cell.8-10 The cell is slightly modified to obtain a two-electrode configuration. When loading gold-coated Si3N4 membranes or PET/ITO sheets, several PTFE spacers (125 µm in thickness each) are added to control the gap between the two gold-coated membranes or PET/ITO sheets as electrode. Two gold foils acting as an ohmic contact are connected to an ammeter. For the open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) tests, a three-electrode configuration is used. A gold-coated Si3N4 membrane acts as the working electrode, a platinum wire and an Ag/AgCl electrode act as the counter and reference electrode, respectively. The beam spot is 3 x 5 mm2. The energy resolution is around 0.65 eV at the Mn K-edge.11 Energy calibration is performed using a Mn foil. The transmission XA spectra at the Mn K-edge are recorded in transmission mode. The transmission signal is measured by an ionic chamber current. The ionic chamber current intensity is proportional to that of the incoming flux. The beam intensity can be varied by changing the slit width. Simultaneously ionic current XA spectra are obtained by directly measuring the drain current between the two gold-coated Si3N4 membranes or PET/ITO sheets using a Keithley ammeter (Model: 6517A) as illustrated in Figure 1 and shown in photo in Figure S1. Unsteady current fluctuation is observed when the liquid is flowing inside the cell. Therefore, the liquid flow is stopped during the ionic current measurements. Each Mn K-edge ionic current or transmission spectrum takes around 6 min.


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Figure 1. Detection schemes of ionic current and transmission measurements under X-ray illumination: A Keithley ammeter with two connections to the front and back electrodes for ionic current detection. An ionization chamber attached behind the second membrane as transmission detector. Two Au/Ti/Si3N4 (30/1-2/150 nm) membranes are used as electrodes for collecting ionic currents. The gap distance between the two electrodes controlled by PTFE sheets and gold foil.

RESULTS AND DISCUSSION 1. Ionic Current Detection Mechanism In this section, experimental parameters including the spacer thickness and the cell orientation are varied to study their effect on the ionic current response upon X-ray irradiation. Note that there is a background ionic current (BIC), which is observed even without X-ray irradiation. This BIC should be ideally as low as possible. Based on these studies, we propose a driving force for the ionic current. 1.1 The influence of PTFE spacer thickness. The PTFE spacer thickness between the two gold-coated Si3N4 membrane electrodes is varied from 125 µm to 500 µm in order to find an optimum gap distance between the two electrodes. Together with the gold foil (127 µm) acting as an ohmic contact, the gap distance between the two membranes that varies between 127 and 627 µm. Figure 2a shows the change of BIC in the cell when changing the PTFE spacer thickness. It


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can be seen that the BIC decreases with increasing gap distance and reaches a steady current at around 250 µm. There is very small beam response (< 100 pA) at this spacer thickness. It is found that only the spacer thickness at around 500 µm gives a large ionic current jump upon chopped Xray irradiation (see Figure 2b). Thus the optimum gap distance is 627 µm. In this case, the BIC without beam is around 16 nA while the ionic current jump with beam at the Mn-edge is around 2-3 nA. It is worth mentioning that the BIC often decays or grows over a long time domain (hours) leading to a background trend in the IC-XAS spectra that can be subtracted afterwards. We thus keep this optimum gap distance for further studies. This BIC can be explained by a small built-in electric potential between the two electrodes. It was previously observed for several metals like Au, Pt, or Cu that their work function changes upon interaction with water.12 Accordingly, the BIC is most likely caused by different surface chemistry happening when the electrodes are immersed into the liquid chamber. beam on

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Figure 2. (a) The relationship between the BIC and the PTFE spacer thickness in the transmission cell. (b) Ionic current and transmission current upon chopped light irradiation study from a 2 mol/liter (M) of Mn(NO 3)2 aqueous solution at energy above Mn absorption edge (at 6.63 keV) with an optimized gap distance of 627 µm.

1.2 Cell orientation and driving force for the ionic current As shown in Figure 1 and Figure S1, our ionic current XA spectra detection setup is different to the ionization chambers which are usually filled with gas or liquid and widely used to detect


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and measure charges created by ionizing radiation.13-15 An ionization chamber works as below: (a) it consist of two conducting electrodes in a sealed container filled with a suitable sensitive media: gas or liquid. (b) Photons are absorbed by the media presenting in the chamber. (c) Positive and negative charge carriers are created by ionization. (d) Charges drift in opposite directions due to the external high voltage (typical in the range of hundred to a few thousands) between the two electrodes. (e) Charges are collected and produce a current that is measured by an electrometer. The current is an indication of the rate of ion-pair formation and collection, and it is proportional to the incident X-ray intensity. In our set-up, aqueous solution is used and there is no external potential applied to the cell. As mentioned above, without beam a BIC that drifts over time exists in the circuit (see Figure 2b). When the beam is switched on, a beam-induced ionic current response is observed that we call here IC (see Figure 2b). When scanning the incoming photon energy across the Mn K-edge, the ionic current shows on the first view a proportionality to the absorption of the Mn K-edge (see later in Figure 4-5). As described above for an ionization chamber, we expect that upon X-ray irradiation positive and negative charge carriers are created in the solution by ionization. Specifically, X-ray-induced radiolysis 16-18 of Mn aqueous solution results in the formation of Mn3+ or Mn4+ cations from the ionization of Mn2+, Auger electrons (eaq-) and free radicals, eg. H* and OH* formed by ionization of water. The higher ionized ions have a higher electrical mobility compared to the lower ionized ones or to neutral molecules.19 However, since we did not apply an external potential, the question is raised of what is the driving force for the observed ionic current. In order to address this question we carried out the following experiment: To monitor the electrode potential change under X-ray irradiation, the open circuit potential (OCP) is recorded using the three-electrode configuration. The OCP of each electrode is measured separately. Figure


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3a shows the OCP change of the 1st and 2nd electrode under chopped beam. The general trend is that without beam, the OCP decreases while with beam it increases. The OCP of the 1st electrode is lower than that of the 2nd electrode. In the next step, we rotated the cell by 180° and carry out the same measurements. After rotation, the OCP of the 1st membrane is now higher than that of that of the 2nd electrode. Note that the OCP difference between the two electrodes with or without beam is less than 50 mV. This potential difference is not sufficient to reduce or oxidize Mn species or water at the working conditions.20 Figure 3b shows the ionic current and the transmission signal changes due to the cell rotation. Initially, with beam on or off, the ionic current signal and the transmission signal response in opposite directions. When the cell is rotated horizontally 180º, they response in the same direction. Based on these observations we propose that a beam-induced electric potential difference between the two electrodes drives the ion transportation. This can be explained as follows: the incoming photons lead to an ejection of photoelectrons from the Si3N4 membrane side or Au side to the air or the electrolyte, respectively (or in the case of Velasco-Velez et al.5 to the vacuum or the electrolyte, respectively). In our cell, the optimized gap distance between the two electrodes is 627 µm and the X-ray beam can reach both electrodes. With this optimum gap distance, X-rays with photon energies around the Mn K-edge (6.54 keV) have 31% transmittance in water. That means that the number of photons hitting an electrode depends on the position of the electrode in the configuration. Since the number of photon reaching the two electrodes differs, the photoelectron loss differs as well. This leads to the different electrical potential of the two electrodes. Based on above results, we propose the following mechanism for ionic current detection in our Mn K-edge spectra: when the incoming photon energy is below the Mn K-edge, the ionization of the atoms and molecules in the solution happens via the Mn L-edge and the K-edges of light atoms,


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eg. N and O. Since in these cases the photon absorption process is off-resonance, the resulting concentration of X-ray-ionized species is relatively low compared to that of at resonance of the Mn K-edge. This is consistent with our observation that below or far above the Mn K-edge, the ionic current jump is around 0.1-0.3 nA when switching the beam on and off. When the incoming X-ray energy is at or a bit above the Mn K-edge resonance, the ionic current jump is around 2-3 nA with the optimum conditions. The ionization of Mn cations lead to a large quantity of ionized Mn cations, photoelectrons or eaq-, radicals and anions. With the help of the beam-induced electric potential, ions drift between the two electrodes and result in a higher ionic current jump/IC-XAS spectrum. 0.4

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Figure 3. The result of cell rotation study from a 1 M of Mn(NO3)2 aqueous solution at energy above Mn absorption edge. (a) OCP of the 1st and 2nd electrode tested in a three-electrode configuration under chopped beam. (b) Chopped ionic current and transmission current time-dependent study tested in a two-electrode configuration. The cell is horizontally rotated 180º at 681 sec.

2. Ionic Current XA Spectra of Mn K-edge. In this section, we study the influence of the anion type, the concentration of the solution and the substrate as well as the beam intensity on IC-XA spectra and compare them to XA spectra obtained in transmission mode. We will also discuss distortions from the spectra obtained in ionic current mode.


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2.1 The influence of anions. In order to study the effect of different anion types on the ionic current response, five Mn aqueous solutions including Mn(NO3)2, MnSO4, MnCl2, Mn(CH₃COO)₂ and KMnO4 are investigated (see Table S1 for more details). The IC-XA and transmission spectra of the Mn K-edge of Mn(NO3)2 and Mn(CH₃COO)₂ aqueous solutions are shown in Figure 4. In Figure 4a, the XA spectra shape and peak position of transmission and ionic current are similar for 1M Mn(NO3)2 aqueous solution. However, compared to the transmission spectrum, for the ICXAS spectrum the high intensity features are reduced in intensity with respect to the low intensity features. A similar situation is observed for the MnSO4 and the MnCl2 aqueous solutions (see Figure S2). We correlate this observation to recombination effects, as will be explained in more detail in section 2.2. It is worth mentioning that ion pairs occur in concentrated solutions of electrolytes. In the concentration of Mn nitrate aqueous solution below 3 M, Mn-nitrato complex (ion-ion pairing) formation is not favorable.21 Mn nitrate dissociates and forms fully hydrate species, [Mn(OH2)6]2+ (aq) and NO3−(aq) in the solvent. This makes them possible to move independently in opposite directions under the influence of electric field.22 For aqueous Mn(CH₃COO)₂, presented in Figure 3b, the ionic current XA spectrum shows a different spectral shape compared to that of the transmission spectrum. Differing from the other investigated Mn salts, Mn(CH₃COO)₂ has a relatively large organic counter anion, CH3COO- with a supposed large stokes radius. The large stokes radius results in a slow ion migration between the two electrodes. To test this concept, Mn K-edge ionic current XAS spectra obtained from Mn acetate aqueous solution are collected with different acquisition times: 1, 4 and 8 sec (see Figure S3a). It is found that a longer acquisition time leads to an intensity increase at the “white line”. The white line refers to an intense absorption peak in the near-edge XAS spectrum. In our control experiments with Mn(NO3)2 solution, the acquisition time shows no effect on ionic current XAS spectra. This


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indicates that the bulky acetate group diffuses slower than other anions studied here. In order to investigate this counter ion dependent effect more in detail we used electrochemical impedance spectroscopy (EIS) and determined the charge transfer resistances of Mn(NO3)2 and Mn(CH₃COO)₂ aqueous solutions at the interface of electrode/electrolyte. Nyquist plots are recorded at the potentials close to OCP (see Figure S3b). It can be seen that each spectrum consists of two distorted semicircles. From the Nyquist plots, the charge transfer resistance (Rct) value 23 of gold-coated Si3N4 membrane electrodes in 1 M of Mn(NO3)2 aqueous solution is calculated from the radius of the semicircle in the plot to be 30 Ω which is half of the one obtained for 1 M of Mn(CH₃COO)₂ aqueous solution. The reason of a higher charge transfer resistance obtained from Mn(CH₃COO)₂ aqueous solution can be attributed to the presence of the bulky acetate anions that have a lower mobility compared to NO3- or SO42- anions, thus preventing charge transfer from ions in the electrolyte to the gold surface. Thus the ion recombination happens more easily compared to small anions, eg. NO3- or Cl-. It is not possible to obtain any ionic current XA spectrum of KMnO4 solution under our experimental conditions. The BIC intensity of KMnO4 solution is 10 µA which is 3 order of magnitude higher than that of Mn(NO3)2 solution (around 16 nA as shown in Figure 2b). The high BIC prevents the detection of any beam response. It is worthy to mention that no beam induced Mn deposition on the membrane is observed by eye on other Mn salts tested in this study. For the further study, we focus on the Mn(NO3)2 solution due to the rather intense ionic current jump.


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Figure 4. Mn K-edge ionic current and transmission XA spectra taken from the flow cell filled with 1 M of (a) Mn(NO3)2 or (b) Mn(CH3COO)2 aqueous solution.

2.2 The influence of concentration. In this section, the influence of the concentration of Mn(NO3)2 in aqueous solution will be discussed. The concentration of Mn(NO3)2 in the aqueous solution is varied from 0.05 to 2 M. It is worth mentioning that the concentration can be further decreased, eg. to 0.01 M, but this requires a longer measuring time. The results are presented in Figure 5 and Figure S4. The peak position of the transmission or ionic current spectra obtained from different concentrations is nearly identical (see Figure 5a-c). However, the peak intensity changes strongly with the changing of solution concentration. To quantify the intensity change, the areas from transmission and ionic current spectra are integrated and shown in Figure 5d. As expected, the integrated areas from the transmission spectra increase when the concentration is increased. For the ionic current spectra, the integrated areas show a strong increase when increasing the concentration from 0.05 to 1 M. This can be explained by the increasing number of ions which can interact with the X-rays and contribute together with the further ionization products to the ionic current. When the concentration increases from 0.5 to 1 M, the integrated area still increases slightly. A further increase of concentration from 1 to 2 M does not lead to an increase of the integrated area but instead to a reduction. Figure 4b shows the normalized spectra at the energy of 6.55 keV with the two features a and b marked at 6.55 keV (white line) and 6.60 keV,


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respectively. It is found that a relative low concentration, e.g. 0.1 M, gives a higher ratio of a/b (see Figure S4). These two observations can be explained by recombination effects. When increasing the concentration at some point a limit is reached where the larger number of ions present in the solution may not further increase the signal but increase the chance of the ion recombination instead. Similar, when due to the strong absorption at the white line a higher concentration of ions is created, recombination of the charges distorts the signal from being proportional to the absorption cross section. Accordingly, a non-linear degradation of the resulting measurement signal is observed.15This means that in order to obtain the optimum signal for ionic current spectra, the concentration of the sample should be optimized first in order to avoid

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Figure 5. Mn K-edge (a-b) ionic current and (c) transmission XA spectra taken from the flow cell filled with Mn(NO3)2 aqueous solutions with different concentrations. (d) Plots of integrated area from transmission and ionic current spectra versus concentration. The experimental Mn K-edge spectra are subtracted to the background and then integrated.


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2.3 The influence of beam intensity. The synchrotron beam intensity is varied to study its effect on the ionic current spectrum. The beam intensity is varied by changing the slit width. As mentioned earlier, the beam intensity is proportional to the ionic chamber current and tested by an ammeter. When the ionic chamber current is 11.2 x10-9 A (the black line), the signal-to-noise ratio is nearly as good as that of transmission spectrum as shown previously. When the beam intensity is reduced by closing the slit, the intensity of ionic current also decreases together with the signalto-noise ratio (see Figure 6a). Note that the variation in beam intensity in the here tested range does not affect recombination effects (see Figure 6b).

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Figure 6. Mn K-edge IC-XA spectra: (a) without and (b) with normalization at 6.70 keV taken from the flow cell filled with 1 M Mn(NO3)2 aqueous solution under different incoming flux. Intensity from top (beam intensity a) to bottom (beam intensity f): 11.2, 6.5, 6.2, 4.8, 3.0 and 1.6 x 10 -9 A.

2.4 The influence of the substrate. In addition, the effect of substrate is studied by replacing gold-coated Si3N4 membranes with PET/ITO sheets. It is still possible to obtain an ionic current spectrum, however, the signal to noise ratio is reduced (see Figure S5). This means that the type of substrate may also play a role in the ionic current spectrum. CONCLUSIONS Potential applications. As previously reported4,5,7, IC-XAS with its high sensitivity to local structure might become a powerful tool for studying various properties of aqueous solutions. It can be used e.g. to study hybrid solvents in Li-ion batteries where the knowledge of ion pairing


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and local solvation structure is essential for optimization of the ionic conductivity and the energy transfer rate.24, 25 Since X-rays cannot be transmitted through the whole cell due to the thickness of the metal electrode which can be in the mm regime, the transmission mode might not be possible anymore. The ionic current jump also can be potentially useful for an aqueous dosimeter working without any external electrical potential. This can be an alternative approach to the currently commercially available liquid ionization detector or dosimeter which works under a high electrical potential (~a few to 150 kV).26 A liquid ionization detector or chamber may potentially become a valuable tool in modern radiation dosimetry, where highly modulated radiation beams are used, since these detectors can be manufactured with a small measurement volume compared to conventional ionization chambers due to its high density and sensitivity.15 Limitations. For possible applications the limitations of this approach need to be carefully considered. As observed experimentally, recombination effects may alter the IC-XAS spectral intensity with respect to the XA transmission spectra. We also observed that the specific counter ion, thus the charge carriers play a role for the detection mechanism. In addition, the performance of these liquid detectors is also affected by a possible non-negligible leakage currents.15 Furthermore it has to be considered that the beam-induced electric field, that we observed here experimentally, attracts and accumulates the ions in the surface region of the electrode or window, leading to a gradient of charges. Note that this X-ray induced charging effect might also have impact on all X-ray measurements where only one membrane is used, thus on most of the X-ray studies on liquid samples. In summary, the mechanism of ionic current and ionic current X-ray absorption spectra of Mn K-edge obtained from a two-electrode flow cell are presented. Based on the results of the spacer thickness variation and the cell orientation, the possible mechanism of the origin of the ionic


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current spectrum is proposed as the beam-induced electric potential difference between the two electrodes. The influence of the type of anions, the concentration, the beam intensity and the type of substrate on Mn K-edge IC-XA spectra is studied. When comparing different anions, the ionic current and transmission XA spectra of Mn(NO3)2 are similar while that of Mn(CH₃COO)₂ aqueous solution are different. Mn(CH₃COO)₂ containing an organic group with large stokes radius may account for the spectrum difference. Since ionic current spectra relay on the ions drifting between the two electrodes, they are affected by the concentration, the surface charge of electrodes and the chemical structure of anions e.g. via recombination effects. ASSOCIATED CONTENT SUPPORTING INFORMATION

The Supporting Information is available free of charge on the ACS Publications website at DOI:…… Additional table (list of a few manganese salts used in this study), the photo of the ionic current measurement set-up used, ionic current spectra at Mn K-edge and EIS data (PDF). AUTHOR INFORMATION Corresponding Author [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS


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We thank I. Rudolph for the preparation of the Au coatings. We thank Dr. D. Többens, Dr. M. F. Tesch and Dr. J. Xiao for their assistance and discussion. This work is supported by the Helmholtz Association (VH-NG-1140). REFERENCES (1) Evangelista, F.; Carravetta, V.; Stefani, G.; Jansik, B.; Alagia, M.; Stranges, S.; Ruocco, A. Electronic Structure of Copper Phthalocyanine: An Experimental and Theoretical Study of Occupied and Unoccupied Levels. J. Chem. Phys. 2007, 126, 124709. (2) Cappa, C. D.; Smith, J. D.; Wilson, K. R.; Saykally, R. J. Revisiting the Total Ion Yield X-ray Absorption Spectra of Liquid Water Microjets. J. Phys.: Condens. Matter 2008, 20, 205105. (3) Wilson, K. R.; Tobin, J. G.; Ankudinov, A. L.; Rehr, J. J.; Saykally, R. J. Extended X-ray Absorption Fine Structure from Hydrogen Atoms in Water. Phys. Rev. Lett. 2000, 85, 4289-4292. (4) Schön, D.; Xiao, J.; Golnak, R.; Tesch, M. F.; Winter, B.; Velasco-Velez, J.-J.; Aziz, E. F. Introducing Ionic-Current Detection for X-ray Absorption Spectroscopy in Liquid Cells. J. Phys. Chem. Lett. 2017, 8, 2087-2092. (5) Velasco-Velez, J.-J.; Wu, C. H.; Pascal, T. A.; Wan, L. F.; Guo, J.; Prendergast, D.; Salmeron, M. The Structure of Interfacial Water on Gold Electrodes Studied by X-ray Absorption Spectroscopy. Science 2014, 346, 831-834. (6) Lawrence Berkeley National Laboratory. Team Reveals Molecular Structure of Water at Gold Electrodes.

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(25) Unger, I.; Seidel, R.; Thurmer, S.; Pohl, M. N.; Aziz, E. F.; Cederbaum, L. S.; Muchova, E.; Slavicek, P.; Winter, B.; Kryzhevoi, N. V. Observation of Electron-Transfer-Mediated Decay in Aqueous Solution. Nat. Chem. 2017, 9, 708-714. (26) PTW Freiburg GmbH, http://www.ptw.de/liquid-filled_ion_chambers.html. (27) Seidel, R.; Ghadimi, S.; Lange, K. M.; Bonhommeau, S.; Soldatov, M. A.; Golnak, R.; Kothe, A.; Könnecke, R.; Soldatov, A.; Thürmer, S. F.; Winter, B.; Aziz, E. F. Origin of Dark-Channel X-ray Fluorescence from Transition-Metal Ions in Water. J. Am. Chem. Soc. 2012, 134, 16001605.


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TOC Graphic


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Ionic Current Mn K-Edge X-ray Absorption Spectra Obtained in a Flow

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