Ultrafast Time-Resolved X-ray Absorption Spectroscopy of

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Ultrafast Time-resolved X-ray Absorption Spectroscopy of Ferrioxalate Photolysis with a Laser Plasma X-ray Source and Microcalorimeter Array Galen C. O'Neil, Luis Miaja-Avila, Young Il Joe, Bradley K. Alpert, Mahalingam Balasubramanian, Dodderi M. Sagar, William Bertrand Doriese, Joseph W. Fowler, Wilfred K. Fullagar, Ning Chen, Gene C. Hilton, Ralph Jimenez, Bruce Ravel, Carl D. Reintsema, Dan R. Schmidt, Kevin L. Silverman, Daniel S. Swetz, Jens Uhlig, and Joel N. Ullom J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b00078 • Publication Date (Web): 17 Feb 2017 Downloaded from http://pubs.acs.org on February 21, 2017

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The Journal of Physical Chemistry Letters

Ultrafast Time-resolved X-ray Absorption Spectroscopy of Ferrioxalate Photolysis with a Laser Plasma X-ray Source and Microcalorimeter Array Galen C. O’Neil,∗,† Luis Miaja-Avila,† Young Il Joe,† Bradley K. Alpert,† Mahalingam Balasubramanian,‡ D. M. Sagar,¶ William Doriese,† Joseph W. Fowler,† Wilfred K. Fullagar,§ Ning Chen,k Gene C. Hilton,† Ralph Jimenez,¶,⊥ Bruce Ravel,# Carl D. Reintsema,† Dan R. Schmidt,† Kevin L. Silverman,† Daniel S. Swetz,† Jens Uhlig,§ and Joel N. Ullom∗,†,@ †National Institute of Standards and Technology, Boulder, Colorado, USA ‡Argonne National Laboratory, Advanced Photon Source, Lemont, Illinois 60439, USA ¶JILA,National Institute of Standards and Technology and University of Colorado Boulder, Boulder, Colorado, USA §Department of Chemical Physics, Lund University, Lund, Sweden kCanadian Light Source, Saskatoon, SK, Canada ⊥Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, Colorado, USA #National Institute of Standards and Technology, Gaithersburg, Maryland, USA @Department of Physics, University of Colorado Boulder, Boulder, Colorado, USA Received February 16, 2017; E-mail: [email protected]; [email protected]

Abstract The detailed pathways of photoactivity on ultrafast timescales are a topic of contemporary interest. Using a tabletop apparatus based upon a laser plasma x-ray source and an array of cryogenic microcalorimeter x-ray detectors, we have measured a transient x-ray absorption spectrum during the ferrioxalate photoreduction reaction. With these high efficiency detectors we observe the Fe K edge move to lower energies, and the amplitude of the extended x-ray absorption fine structure reduce, consistent with a photoreduction mechanism in which electron transfer precedes disassociation. These results are compared to previously published transient x-ray absorption measurements on the same reaction, and found to be consistent with the results from Ogi et al and inconsistent with the results of Chen et al . 1,2 We provide quantitative limits on the Fe-O bond length change. Finally, we review potential improvements to our measurement technique, highlighting the future potential of tabletop x-ray science using microcalorimeter sensors.

Ferrioxalate [FeIII (C2 O4 )3 ]3− complexes exist in natural waters including aerosols. The photoreduction of ferrioxalate is often expressed as 2[FeIII (C2 O4 )3 ]3− + hν → 2[FeII (C2 O4 )2 ]2− + 2CO2 + C2 O2− 4 . This reaction produces CO2 •− radicals as intermediates that go on to initiate other chemical reactions. For example, it is thought that photolysis of these complexes is one of the main sources of active oxygen species (• OH, HO2 • , H2 O2 ), where the formation of the species is catalyzed by FeII and FeIII ions in Fenton-like reactions and this photolysis is responsible for the consumption of oxygen in natural waters 3,4 Ferrioxalate is also used as a chemical actinometer, providing a chemical method of measuring light intensity. 3 The literature on ferrioxalate proposes two mechanisms by which photoreduction proceeds: 1) a prompt reduction

mechanism which has FeII intermediates, believed to be [FeII (C2 O4 )2 ]− +CO2 +CO2 •− at 100 ps and 2) a slow reduction mechanism which has FeIII intermediates, believed to be [FeIII (C2 O4 )2 ]− +2CO2 •− at 100 ps. Figure 1 shows the structure of ferrioxalate and the intermediate in the prompt case. An important component of the debate over the photoreduction mechanism in ferrioxalate is the observation of very different time-resolved x-ray absorption spectroscopy signatures by Chen et al and Ogi et al . 1,2 X-ray Absorption Spectroscopy (XAS) is a powerful tool that enables element-specific measurements of electronic and geometric structure. The oxidation state of the absorbing element is revealed by a “chemical shift”, meaning a translation of the absorption edge feature in energy. Geometric structure is studied by analyzing the Extended X-ray Absorption Fine Structure (EXAFS), which appears as oscillatory features in x-ray absorption over a few hundred eV span above an absorption edge. Time-resolved XAS, where x-ray absorption spectra are measured shortly after photoexcitation of the sample, is ideal for determining the oxidation state and structure of intermediate photo-products in the photoreduction of ferrioxalate. Chen et al report timeresolved XAS measurements showing a large reduction in the Fe-O bond length, and do not discuss the edge shift. Chen et al compare the Fe-O bond length in the intermediate product to the length measured in many Fe containing compounds compounds, and find it to be more consistent with FeIII than FeII . 2 On the other hand, Ogi et al report time-resolved XAS measurements taken with the SACLA free electron laser which show little to no change in the FeO bond length, and show a -4 to -5 eV edge shift within 140 fs. Ogi et al argue for FeII intermediates based upon comparison of the observed edge shift to density function theory (DFT) calculations. 1,5 Ogi et al provide a more detailed discussion of the literature on this topic, including discussion of evidence from time-resolved radical scavenging experiments, some of which point to FeII intermediates as well. 1,3,6 Here we report independent time-resolved XAS measurements on [FeIII (C2 O4 )3 ]3− using a tabletop laser driven plasma x-ray source and microcalorimeter array detectors. Our measurements were performed at a single

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mission, which is planned for launch in the late 2020s with a 4000 pixel microcalorimeter array x-ray spectrometer. 28 While increased array size is desirable, the fact that we intentionally limited the count rate shows that the collection area and quantum efficiency are already sufficient. Instead, the most direct route to improvement for time-resolved XAS is operation at higher count rates per pixel. Eliminating the energy resolution degradation due to crosstalk would improve energy resolution and increase count rates by a factor of 3. With the use of analysis algorithms designed for high count rates, this factor could increase to 10. 29 Count rates can be further increased by the use of high bandwidth multiplexing techniques such as microwave multiplexing, allowing the readout of pixels with faster thermal recovery and therefore higher maximum count rates. 30,31 Other optimizations, including changing the sample interaction geometry and using an x-ray optic with a smaller spot size, could lead to similarly large gains. Hence, there is a large amount of room for further improvements with which to make time-resolved XAS with microcalorimeters and laser plasma based x-ray sources a very powerful technique. While the sensitivity of tabletop time-resolved XAS techniques is likely to always lag the most capable large facilities, here, we show that tabletop techniques can already resolve open questions about intermediate state dynamics. In addition to making picosecond x-ray analysis accessible to a wider circle of researchers, tabletop techniques may enable more efficient allocation of beam-time at large facilities by pre-screening samples of interest. Acknowledgement The authors thank Eleanor Waxman for useful discussions, and thank Ilari Maasilta and his group at Jyv¨ askyl¨ a University (Finland) for their international participation in the connection of microcalorimeter sensors to tabletop time resolved x-ray science. The authors gratefully acknowledge financial support from the NIST Innovations in Measurement Science program and US Department of Energy - Basic Energy Sciences. Sector 20 facilities at the Advanced Photon Source are supported by the US Department of Energy - Basic Energy Sciences and the Canadian Light Source. Supporting Information. Experimental apparatus figure, microcalorimeter calibration, sample preparation, polycapillary optic, BM20 configuration, χ e(R) signal-to-noise, fitting model detail, cause of EXAFS amplitude reduction, Fe-O bond length change energy space method, excitation fraction compared to photons per molecule.

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