Relaxation of Electronically Excited Hydrogen Peroxide in Liquid

Article pubs.acs.org/JPCC

Relaxation of Electronically Excited Hydrogen Peroxide in Liquid Water: Insights from Auger-Electron Emission Stephan Thürmer,† Isaak Unger,† Petr Slavíček,*,‡ and Bernd Winter*,† †

Helmholtz-Zentrum Berlin für Materialien und Energie, and BESSY D-12489 Berlin, Germany Department of Physical Chemistry, Institute of Chemical Technology, Technická 5, 16628 Prague, and J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejškova 3, 18223 Prague 8, Czech Republic

‡

ABSTRACT: Autoionization electron spectroscopy is applied to study nonradiative relaxation processes of hydrogen peroxide aqueous solution irradiated by soft X-rays. The high kinetic energy part of the oxygen 1s H2O2(aq) Auger-electron spectrum reveals dicationic final states with considerably lower energy than for neat liquid water. Assisted by quantum chemical calculations, it is argued that such lower-energy states arise from two fundamentally different relaxation processes. One is (local) Auger decay, yielding H2O22+(aq) species, and here the low final-state energy arises from charge delocalization across the molecular O−O bond. Alternatively, nonlocal dicationic states can form, corresponding to a charge-separated complex comprising H2O2 and a neighboring water molecule. Different charge-separation mechanisms, depending on whether or not proton dynamics of the core-level excited or ionized H2O2 molecule is involved, are discussed. We also present for the first time the partial electron yield X-ray absorption spectrum of liquid water, which is useful in interpreting the respective spectra from H2O2 in water, especially when identifying solute-specific excitations.

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photoelectron spectra measured near the O 1s → LUMO (lowest unoccupied molecular orbital) electronic X-ray transition. Previous XA measurements from liquid water were based on total ion, electron, and fluorescence yields,21,22 and more recently absorption spectra were also obtained from X-ray emission spectroscopy.23−25 Electron detection measurements from volatile liquid solutions are scarce because they are more difficult to perform than photon-out detection. Due to the large electron scattering cross sections emitted photoelectrons lose some fraction of their kinetic energy through collisions with gas-phase water molecules on their way to the electron detector. An accurate determination of the initial energy of the (photo)electrons is thus not possible. This problem has been recently resolved, though, by the development of the vacuum liquid microjet technique,26,27 which is now becoming a standard tool for spectroscopy of volatile liquids. This technique is also applied in the present study. With the recorded Auger-electron spectra from aqueousphase H2O2 in this study we explore the relaxation channels of hydrogen peroxide upon X-ray irradiation, and we discuss possible implications for aqueous-phase structure, in particular the arrangement of the hydrogen bonds. Hydrogen peroxide is arguably the closest structure analogue to water and easily integrates into water’s hydrogen-bonding network.28−30 It is thus interesting to compare electronic-structure interactions of

INTRODUCTION Hydrogen peroxide is an important oxidation species in industrial chemistry,1−3 physiology,4,5 atmospheric, and planetary processes.6−9 It is also one of the most abundant products and intermediates in water radiolysis,10−13 and thus full spectroscopic characterization of hydrogen peroxide in aqueous environment is important. X-ray-based spectroscopies are particularly desirable as they are sensitive to a local atomic environment, allowing for discernment of identical atoms in different molecular species,14,15 for instance oxygen in the various reactive species that form upon water irradiation. Surprisingly, the amount of experimental electronic-structure information on hydrogen peroxide is still rather limited. Valence photoelectron (PE) spectra of gas-phase hydrogen peroxide have been measured in the 1970s,16,17 and aqueousphase H2O2(aq) photoelectron spectra for both valence and core electrons were reported by our group only recently.18 The X-ray absorption spectrum of H2O2 was previously recorded in the gas phase,19 from water ice films,20 but not yet from the aqueous phase. Autoionization (Auger) spectroscopy, by which both ultrafast relaxation processes and the involved species can be identified through electron kinetic energy, is not known for hydrogen peroxide in any state (gas phase, dilute water solution, pure hydrogen peroxide solution, and solid hydrogen peroxide). Here, we report for the first time the Auger-electron spectra from hydrogen peroxide in liquid water, for 1.0−29.4 m solutions. Our work also presents the very first partial-electronyield (PEY) X-ray absorption (XA) spectrum from liquid water (and also from H2O2(aq)), i.e., obtained from the respective © 2013 American Chemical Society

Special Issue: Ron Naaman Festschrift Received: February 13, 2013 Revised: May 30, 2013 Published: June 3, 2013 22268

dx.doi.org/10.1021/jp401569w | J. Phys. Chem. C 2013, 117, 22268−22275

The Journal of Physical Chemistry C

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H2O2(aq) with well-studied H2O(aq),21,23−25,31−35 and here we focus on the nonradiative energy relaxation, i.e., the autoionization channels of oxygen 1s core-level ionized/excited H2O2 in liquid water. The dominant mechanism is Augerelectron decay, a local and very fast process (on the time scale of the 4 fs lifetime of the oxygen core hole32,33), involving only the photoionized or photoexcited species, i.e., H2O2 molecule in our case. One of the valence electrons refills the core hole, and the energy released is used to eject another valence electron from the same molecule. For ionization the product is thus dicationic H2O22+(aq). In the gas phase this is the only autoionization channel, but in the liquid phase also nonlocal variants of the Auger processes are possible, involving the ultrafast electron and charge transfer to the solvating water molecules. Such nonlocal processes can be revealed through the occurrence of an extra electron intensity at higher kinetic energy than the leading Auger-electron peak, as was recently shown for liquid water.36 The final species still has two electron holes, but shared between neighboring water molecules,37 and the lower total energy of the final state (and correspondingly higher kinetic energy of the ejected electrons) is due to Coulomb interaction of the two charges which are now separated by a larger distance.36,37 This specific type of nonlocal autoionization process, which leads to charge delocalization between two weakly interacting molecular or atomic species, is called Intermolecular Coulombic Decay (ICD).38,39 ICD upon ionization can be thus viewed as a process in which the energy released in the core or inner-valence level refill is used to ionize a weakly interacting neighboring molecule. For core-level ionization of liquid water, this leads to the formation of a pair of reactive water cations, H2O+···H2O+,40 quickly engaging in chemical reactions and being possibly triggered by an initial Coulomb explosion.41 We have recently identified additional de-excitation mechanisms in liquid water, combining nuclear motion along the hydrogen-transfer coordinate with a subsequent (local) Auger or ICD decay,36 even on the 4 fs time scale of the core-hole lifetime. These so-called protontransfer-mediated charge separation (PTM-CS) processes are potentially present for all hydrogen-bonded systems, being most efficient when the O−H covalent bond and the hydrogen bond are oriented along the same axis. Analyzing hydrogen bonding in aqueous solution through PTM-CS is thus tempting. Here, we are specifically asking whether the PTMCS can be also identified in hydrogen peroxide in water. These processes would need to be distinguished from delocalization processes not requiring nuclear dynamics, which also depend on the hydration structure details. H2O2, having more than one non-hydrogen atom, represents the simplest complex molecule where the novel PTM-CS relaxation processes can be studied in aqueous solution. The relaxation processes in hydrogen peroxide should be similar to those found in water. There are several important differences between water and hydrogen peroxide, though. First, the two oxygen atoms of H2O2 allow for intramolecular charge separation across the O−O bond. Second, hydrogen peroxide is a highly flexible molecule due to the weakly hindered rotation of the OH groups about the O−O bond. This (dihedral) angle between the OH groups does in fact vary over a large range which leads to a wide distribution of hydration configurations.30 Third, hydrogen peroxide is easily incorporated into the water hydrogen-bond network; the hydrogen-peroxide molecule is actually a better hydrogenbond donor than water.28,30 The structure of the hydrogen-

bond network is however different, and a perfect tetrahedral arrangement is not necessarily achieved. Hence, the direction of covalent and hydrogen bonds may not be favorable for PTMCS, unlike in neat liquid water.28 These various processes will be discussed here, based on experimental autoionization/Auger spectra, and assisted by quantum chemical calculations. Our calculations provide the Auger-electron energies of gas-phase H2O2 (surprisingly, experimental values are unavailable), and we also report on calculated total energies for the core-ionized hydrogen peroxide molecule along the O−H stretching coordinate for the H2O2··· H2O and H2O2···(H2O)2 hydrogen peroxide−water complexes.

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METHODS Experimental Section. Photoelectron spectroscopy measurements were performed from a 15 μm sized vacuum liquidwater jet27,42 at the soft-X-ray U41-PGM undulator beamline of BESSY II, Berlin. The jet velocity was ∼80 ms−1, and the jet temperature was 6 °C. Electrons were detected with a hemispherical electron analyzer, separated by a 100 μm diameter orifice from the liquid jet at a distance of approximately 0.3 mm. Detection was at normal direction with respect to the synchrotron-light polarization vector. The energy resolution of the U41 beamline is better than 200 meV at the incident photon energies used here, and the resolution of the hemispherical energy analyzer is constant with kinetic energy (about 200 meV, at 20 eV pass energy). Small X-ray focal size, 23 × 12 μm2, assures less than 5% gas-phase signal to the total photoelectron signal. Hydrogen peroxide aqueous solutions were prepared using deionized water to which amounts of H2O2 aqueous solutions (50 wt % in H2O) were added to yield 1.0−29.4 m concentrations. At 29.4 m concentration H2O2 molecules are inevitably in contact with each other, and even at 6 m concentration few H2O2 molecules will be within the first solvation shell of H2O2.30 For the photoelectron measurements a small amount of NaCl (yielding 10 eV above the ionization threshold. This is not true for the 29.4 m solution, and the scaling may not be as meaningful; this case is not discussed though in the present work as our focus is on H2O2−H2O interactions (see text). In (B) the water contribution was subtracted to single out the H2O2 contribution to the XA spectrum.

all concentrations we find an almost identical absorption band for H2O2(aq), with maximum at approximately 533 eV photon energy, that is much lower than for H2O. This energy separation between H2O and H2O2 states thus enables specific excitation of the solvated hydrogen peroxide molecule. Even for the 29.4 m solution when H2O2−H2O2 interactions are abundant30 the H2O2 band does not change noticeably; this very high concentration will not be further considered, though. Band position, width, and overall shape are very similar to the electron-energy-loss spectrum from gas-phase H2O2 where the maximum of the first peak is also found at 533 eV.19 The observed peak position of H2O2 in water is also consistent with the assignment for H2O2 formed upon ice radiolysis.20 The almost zero solvatochromic shift of the XA spectra, i.e., the constant 533 eV band position with concentration, is in good agreement with the above assumption that solute and solvent 22271



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We next attempt to interpret the ionization-induced autoionization spectrum of H2O2 in water, which is difficult and inevitably speculative, although we believe that our arguments are rather substantial. A good starting point is to compare with the fairly well-understood spectrum of neat water (blue line in Figure 4A). Here peak 1 at 504 eV KE is the leading K-1b11b1 normal Auger peak, i.e., the highest energy state of H2O2+(aq) as was already mentioned. Because of solvent effects this peak is shifted to approximately 4.5 eV higher KE than in the gas phase; a similarly large energy shift occurs for all other Auger peaks as well.32,37 The large shoulder 2 in the water autoionization spectrum at approximately 508.5 eV KE has no analogue for gas-phase water and is due to the charge-delocalized states, as in H2O+···H2O+ created within the ICD process or as in HO+···H+···H2O and HO···H+···H2O+ structures created in the PTM-CS process.36 Unfortunately, we cannot fully pursue the analogous procedure to analyze H2O2(aq) because the gas-phase Auger spectrum of H2O2 is not available. We instead base our examination on Augerelectron energies calculated by quantum chemical methods; see bars in Figure 4B. Let us start with a qualitative consideration of the spectral differences between hydrated hydrogen peroxide and a hydrated water molecule. As a very first guess one might expect that the Auger spectrum of H2O2(aq) has an overall similar shape to the respective Auger spectrum from neat water, but extending to somewhat larger KEs. This would follow from the fact that the valence and core-level PE spectra of the two solutions are very similar; however, the lowest ionization energy of H2O2(aq) is 0.5 eV smaller, and the O 1s binding energy is approximately 1 eV higher than for liquid water.18 In the simplest estimate, and neglecting Coulomb interactions, the Auger spectrum of H2O2 would thus be altogether shifted to 2 eV higher KE, but this is obviously not the case. Not only is the energy shift considerably larger (Figure 4A), with the H2O2(aq) spectrum extending approximately 6 eV beyond the H2O(aq) spectrum, but also the structure of the H2O2(aq) spectrum is richer, exhibiting a maximum at peak a (at 506 eV) and two high-KE shoulders b (510.0 eV) and c (514.0 eV). The existence of such higher-KE features indicates that the doubleionization potentials of hydrogen peroxide are of a lower energy compared to water; the core-ionization energies of both species are similar, differing by only 1 eV, as we have seen. The final states can be of local or nonlocal character, but such a distinctionbased on experimental groundsis not as obvious for polyatomic H2O2 in water as it is for neat water. Intramolecular charge delocalization across the O−O covalent bond is likely to lead to comparable energy lowering as for nonlocal charge delocalization between H2O2 and H2O molecules (H2O2−H2O2 interactions, which will play a role at the higher concentration (6 m), cannot be resolved in the experiment and are not explicitly considered here). For a first test of this assumption it is instructive to compare the H2O2 spectrum of Figure 4B with that of an oxygen molecule56 and of solid oxygen;57 these are probably the best available substitutes for the Auger spectrum of gas-phase H2O2. The O2 Auger spectrum56,58 is found to indeed resemble the one measured here, exhibiting a very similar three-peak structure at almost the same energies. Moreover, O2 gas- and solid-phase spectra are fairly similar, in agreement with our findings here for the liquid phase. Also our quantum chemical analysis shows that dicationic energies of gas-phase H2O2, H2O22+, are considerably lower than for H2O2+, arising from the across-the-bond

we scale the Auger spectrum to compensate for the decrease of water molecules in a given volume; i.e., here we neglect the very small H2O2 signal. The resulting differential spectrum is presented in Figure 4A (in green); we also show the water

Figure 4. Auger-electron spectra of H2O2 aqueous solution and of neat liquid water obtained well above the ionization (A), using 600 eV photon energy, and at the characteristic XA pre-peaks, i.e., 533 and 535 eV photon energy (B), respectively. The H2O2 excitation autoionization spectrum from solution (B) is presented as measured; concentration was 6 m. The ionization autoionization spectrum was obtained by subtracting the water contributions. Spectra from neat water and from 6 m H2O2 aqueous solution, scaled as described in the text, are presented for reference. Two approaches were used for subtraction, one for 3 m and another for 6 m H2O2 aqueous solution, as described in the text. Peak labels and shifts (indicated by vertical dashed lines) are explained in the text. Calculated Auger energies corresponding to singlet states are shown as blue (isolated water) and red (isolated hydrogen peroxide) bars in (A). Calculated energies were shifted by 4.5 eV to higher kinetic energy to account for long-range solvent polarization. Intensities of the H2O2 spectra in (A) were scaled with respect to the water molar fraction, whereas intensities in (B) are displayed to yield same maximum peak height.

Auger-electron spectrum for comparison. In our second approach we adhere to the intensity difference found in the XA spectrum between H2O2(aq) and neat water (red and blue spectrum in Figure 3). The Auger spectrum, this time from 6 m solution, also measured at 600 eV photon energy, was scaled to yield the same intensity ratio between water and H2O2(aq) at high photon energy as in Figure 3. The resulting differential spectrum yields the red curve in Figure 4A; it is almost a replica of the 3 m spectrum which gives us sufficient confidence in our analysis. There will be differences due to changes in bonding structure for these different concentrations, though, but these effects seem to merely affect the spectra. 22272



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covalent bond and hydrogen bond, O−H···O, are arranged almost collinearly. This is very similar to liquid water36 where the PTM-type processes were observed.36 Note that according to our calculations the gas-phase H2O2 molecule insignificantly changes its geometry upon core ionization. The nuclear rearrangement occurring in aqueous solution is thus only possible due to the interaction of H2O2 with neighboring water (and possibly also with hydrogen peroxide) molecules. In fact, as Figure 5 shows, the geometry of the hydrogen-bond configuration is a means to control PTM-type processes. As mentioned above, an unequivocal experimental identification of lower-lying states arising from PTM-CS is currently not possible, though, because of very similar final-state energies for intramolecular delocalization. Some additional insight into the core-hole relaxation mechanism can be obtained when inspecting the resonant autoionization spectra of H2O2(aq). The resonance Auger spectrum from 6 m concentration obtained at 533 eV photon energy (absorption band maximum) is shown in Figure 4B (red curve). Here, we also present the analogous water excitation− autoionization spectrum (blue curve), measured at liquid water’s first absorption band (535 eV). Observed energy shifts with respect to the ionization spectrum are connected with the occurrence of the above-mentioned spectator and participator Auger decays; see also ref 32 for a detailed discussion. The dashed vertical lines in Figure 4 mark the spectator Auger energy shift for the leading peak (compare figure caption). Participator processes give rise to the highest KE signal. Probably, the most important observation from Figure 4 is that also the excitation-Auger spectrum from hydrogen peroxide aqueous solution exhibits the three-peak structure (now labeled a′, b′, c′) found for ionization Auger decay. Hence, it is likely that the relaxation processes are essentially the same for the two cases, and the effect of the extra valence electron (note that the final state is singly charged, i.e., 1 electron and 2 holes, unless the electron delocalizes within the core-hole lifetime) is rather small.

delocalization in a hydrogen peroxide molecule. Calculated Auger energies are presented at the bottom of Figure 4A for both water (blue bars) and hydrogen peroxide (red bars). Since solvent polarization shifts should be approximately the same (compare Methods)both molecules are in waterwe have shifted all calculated energies to 4.5 eV higher KE, a value found previously for liquid water.32 The lowest energy dicationic states are at much lower energies compared to water and hence give rise to a signal at larger KE. The density of states implies that the three-peak structure for H2O2(aq) can be rationalized without considering nonlocal electronic decay channels. However, the highest computed electron KE (including the estimated shift due to long-range polarization effects) is still some 2 eV below the experimental energy of peak c. Both observations lead us to the tentative conclusion that peak c may well arise from nonlocal contributions, either from ICD in the ground-state molecular structure or from PTM-CS processes of the type observed for neat liquid water.36 This may be also true for peak b, but we cannot judge from the experimental data. We have investigated the relevance of the proton-transfermediated processes upon core-level ionization with quantum chemical calculations. Figure 5 shows energy profiles for core-

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CONCLUSIONS Resonant and nonresonant autoionization/Auger spectra, as well as partial electron yield X-ray absorption spectra, were recorded for various H2O2 concentrations in water. Using photo/Auger-electron spectroscopy we have examined for the first time the character of species occurring during the primary events of energy dissipation upon the hydrogen peroxide coreionization in liquid water. On the basis of the autoionization spectra, which were compared with measurements from neat liquid water, we have discussed possible ultrafast relaxation processes following the core ionization. The observed fewelectron volts larger kinetic energies in the ionization Auger electron spectra from H2O2(aq) as compared to H2O(aq) indicate charge delocalization in the final dicationic states. Two processes were considered, across-the-bond (still localized on the molecule) and across-the-molecules (nonlocal) charge separation, having no analogue in the gas phase. While there are indications that the nonlocal electronic states are indeed populated, the experimental evidence is not fully conclusive as the H2O22+ and H2O2+···H2O+ dicationic species are relatively close in energy. This can be contrasted with water where the double positive charge is concentrated on a single oxygen atom in H2O2+. The charge-delocalized H2O+···H2O+ complexes are then energetically well separated.

Figure 5. Energy profiles along the O−H stretching coordinate for H2O2···H2O (blue color) and H2O2···(H2O)2 (red color) complexes. Calculations performed at the HF level with frozen core orbitals, using the cc-pCVTZ basis for oxygen and cc-pVTZ basis for hydrogen atoms.

ionized hydrogen peroxide molecules along the O−H stretching coordinate for the two smallest complexes of hydrogen peroxide with water, H2O2···H2O and H2O2··· (H2O)2. These two configurations are argued to represent typically occurring, favorable, and unfavorable PTM situations in hydrogen peroxide aqueous solution. Recall that the oxygen core-hole lifetime is extremely short (4 fs), and therefore only the O−H stretching motion is relevant within this time window. It can be seen in Figure 5 that for the H2O2···H2O complex proton transfer is energetically favorable, yet there is a significant energy barrier preventing the motion on the ultrafast time scale considered. For H2O2···(H2O)2, on the other hand, proton transfer is readily possible, with no barrier, because the 22273



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(3) Thiel, W. R. New Routes to Hydrogen Peroxide: Alternatives for Established Processes? Angew. Chem., Int. Ed. 1999, 38 (21), 3157− 3158. (4) Beckman, K. B.; Ames, B. N. The Free Radical Theory of Aging Matures. Physiol. Rev. 1998, 78 (2), 547−581. (5) Giorgio, M.; Trinei, M.; Migliaccio, E.; Pelicci, P. G. Hydrogen Peroxide: A Metabolic by-Product or a Common Mediator of Ageing Signals? Nat. Rev. Mol. Cell Biol. 2007, 8 (9), 722A−728. (6) Carlson, R. W.; Anderson, M. S.; Johnson, R. E.; Smythe, W. D.; Hendrix, A. R.; Barth, C. A.; Soderblom, L. A.; Hansen, G. B.; McCord, T. B.; Dalton, J. B.; et al. Hydrogen Peroxide on the Surface of Europa. Science 1999, 283 (5410), 2062−2064. (7) Clancy, R. T.; Sandor, B. J.; Moriarty-Schieven, G. H. A Measurement of the 362 GHz Absorption Line of Mars Atmospheric H2O2. Icarus 2004, 168 (1), 116−121. (8) Newman, S. F.; Buratti, B. J.; Jaumann, R.; Bauer, J. M.; Momary, T. W. Hydrogen Peroxide on Enceladus. Astrophys. J. 2007, 670 (2), L143−L146. (9) Cooper, P. D.; Johnson, R. E.; Quickenden, T. I. Hydrogen Peroxide Dimers and the Production of O2 in Icy Satellite Surfaces. Icarus 2003, 166 (2), 444−446. (10) Ferradini, C.; Jay-Gerin, J. P. Radiolysis of Water and Aqueous Solutions - History and Present State of the Science. Can. J. Chem. 1999, 77 (9), 1542−1575. (11) Ershov, B. G.; Gordeev, A. V. Model for Radiolysis of Water and Aqueous Solutions of H2, H2O2 and O2. Radiat. Phys. Chem. 2008, 77 (8), 928−935. (12) Roth, O.; LaVerne, J. A. Effect of pH on H2O2 Production in the Radiolysis of Water. J. Phys. Chem. A 2011, 115 (5), 700−708. (13) Meesungnoen, J.; Filali-Mouhim, A.; Snitwongse, N.; Ayudhya, N.; Mankhetkorn, S.; Jay-Gerin, J. P. Multiple Ionization Effects on the Yields of HO2/O2− and H2O2 Produced in the Radiolysis of Liquid Water with High-LET 12C6+ Ions: A Monte-Carlo Simulation Study. Chem. Phys. Lett. 2003, 377 (3−4), 419−425. (14) Hüfner, S. Photoelectron Spectroscopy: Principles and Applications; Springer-Verlag: Berlin, Heidelberg, New York, London, Paris, Tokyo, Hong Kong, Barcelona, Budapest, 1995. (15) Stöhr, J. NEXAFS Spectroscopy; Springer Verlag: Berlin, 1992. (16) Osafune, K.; Kimura, K. Photoelectron Spectroscopic Study of Hydrogen Peroxide. Chem. Phys. Lett. 1974, 25 (1), 47−50. (17) Ashmore, F. S.; Burgess, A. R. Study of Some Medium Size Alcohols and Hydroperoxides by Photoelectron-Spectroscopy. J. Chem. Soc., Faraday Trans. 2 1977, 73, 1247−1261. (18) Thürmer, S.; Seidel, R.; Winter, B.; Oncak, M.; Slavicek, P. Flexible H2O2 in Water: Electronic Structure from Photoelectron Spectroscopy and Ab Initio Calculations. J. Phys. Chem. A 2011, 115 (23), 6239−6249. (19) Rü hl, E.; Hitchcock, A. P. Oxygen K-Shell Excitation Spectroscopy of Hydrogen Peroxide. Chem. Phys. 1991, 154 (2), 323−329. (20) Laffon, C.; Lacombe, S.; Bournel, F.; Parent, P. Radiation Effects in Water Ice: A near-Edge X-Ray Absorption Fine Structure Study. J. Chem. Phys. 2006, 125 (20), 204714. (21) Wernet, P.; Nordlund, D.; Bergmann, U.; Cavalleri, M.; Odelius, M.; Ogasawara, H.; Naslund, L. A.; Hirsch, T. K.; Ojamae, L.; Glatzel, P.; et al. The Structure of the First Coordination Shell in Liquid Water. Science 2004, 304 (5673), 995−999. (22) Smith, J. D.; Cappa, C. D.; Messer, B. M.; Cohen, R. C.; Saykally, R. J. Response to Comment on ″Energetics of Hydrogen Bond Network: Rearrangements in Liquid Water″. Science 2005, 308 (5723), 793b. (23) Fuchs, O.; Zharnikov, M.; Weinhardt, L.; Blum, M.; Weigand, M.; Zubavichus, Y.; Bar, M.; Maier, F.; Denlinger, J. D.; Heske, C.; et al. Isotope and Temperature Effects in Liquid Water Probed by XRay Absorption and Resonant X-Ray Emission Spectroscopy. Phys. Rev. Lett. 2008, 100 (2), 027801. (24) Tokushima, T.; Harada, Y.; Takahashi, O.; Senba, Y.; Ohashi, H.; Pettersson, L. G. M.; Nilsson, A.; Shin, S. High Resolution X-ray

Further work on both the theory and experimental side is needed to unequivocally identify nonlocal processes, in particular proton-transfer-mediated nonlocal relaxation processes, which can be revealed via isotope substitution experiments, studying D2O2 molecules in D2O liquid water. Such experimentally demanding measurements are currently underway in our laboratory, and whether PTM relaxation processes can indeed be exploited to probe hydrogen-bonding structure remains somewhat unclear. On the theory side, the relative importance of local and nonlocal Auger processes as well as the role of nuclear dynamics assisted charge delocalization will be studied for this important model system. One question to be specifically addressed in upcoming calculations is how the initial arrangement of the hydrogen bonds affects X-ray-induced nuclear motion along the O−H···O bond. To this end, H2O2 is a very interesting test model as it, unlike liquid water, does not typically possess the tetrahedral configuration. Finally, one has to consider the rotational degree of freedom, which may also control the subsequent proton transfer. Since most of the data on ICD-type processes have been so far obtained for atomic systems, in particular for rare gas clusters, an understanding of these processes in small hydrogenbonded polyatomic molecules is of large importance. In parallel to further advancing our understanding of H2O2 in water, we aim at studying mixed hydrogen-bonded systems such as NH3···H2O and also mixed systems containing solutes with multiple heavy, i.e., non-hydrogen atoms, for instance amino acids. The present study of hydrogen peroxide aqueous solution has also shown that ionization-induced relaxation processes are quite similar to excitation-induced autoionization processes. This analogy has a great application potential. Selective X-ray excitation of a hydrogen-bonded solute can thus serve for spatial control of chemical reactions, e.g., oxygen release,59 or triggering local and nonlocal relaxation processes at a particular atomic site, at a given time. The potential of such control schemes for industry, research, and medicine is obvious.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (P.S.); bernd.winter@ helmholtz-berlin.de (B.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Robert Seidel for his contributions to some of the measurements. B.W. acknowledges support by the Deutsche Forschungsgemeinschaft (DFG) via project WI 1327/3-1 and FOR 1789. P.S. acknowledges the support of the Grant Agency of the Czech Republic, project no. 13-34168S.

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REFERENCES

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Relaxation of Electronically Excited Hydrogen Peroxide in Liquid

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