Ultrafast Proton and Electron Dynamics in Core-Ionized Hydrated

Article pubs.acs.org/JPCC

Ultrafast Proton and Electron Dynamics in Core-Ionized Hydrated Hydrogen Peroxide: Photoemission Measurements with Isotopically Substituted Hydrogen Peroxide Isaak Unger,† Stephan Thürmer,‡ Daniel Hollas,§ Emad F. Aziz,†,⊥ Bernd Winter,*,† and Petr Slavíček*,§ †

Joint Laboratory for Ultrafast Dynamics in Solutions and at Interfaces (JULiq), Helmholtz-Zentrum Berlin, Albert-Einstein-Strasse 15, D-12489 Berlin, Germany ‡ Center for Frontier Science (CFS), Chiba University, 1-33 Yayoi-cho, Inage, Chiba 263-8522, Japan § Department of Physical Chemistry, Institute of Chemical Technology, Technicka 5, 16628 Prague, Czech Republic ⊥ Department of Physics, Freie Universität Berlin, Arnimallee 14, D-14159 Berlin, Germany S Supporting Information *

ABSTRACT: Auger-electron spectroscopy is applied to hydrogen peroxide aqueous solution to identify ultrafast electronic relaxation processes, specifically those involving a proton transfer between core-ionized hydrogen peroxide and solvating water molecules (proton transfer mediated-charge separation, PTM-CS). Such processes yield dications where the two positive charges resulting from the Auger decay are delocalized over the two molecules. These species contribute to the high-energy tail of the Augerelectron spectrum as do also species resulting from charge delocalization in the ground-state geometry. However, the immediate and secondary transient species are different for ground-state and proton-transferred structures. Here we show that it is possible to experimentally distinguish the species by studying the H2O2/D2O2 isotope effect on the Auger spectra. To interpret the measured Auger-electron spectra, we complement the experiment with ab initio based dynamical calculations.

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electrons.9,14−20 Despite the fact that nonradiative decay is by far more probable than X-ray fluorescence, the former is the less studied process because of the large scattering cross sections of electrons with water vapor.18,21 This hurdle is circumvented by the use of a vacuum liquid microjet, which is exposed to monochromatic high-intensity X-rays from an undulator beamline.18,22 For the present study of hydrogen peroxide in water, aiming at obtaining an explicit spectroscopic fingerprint of transient (reactive) species, we detect electrons arising from (local) Auger18 as well as from other nonlocal autoionization channels, as we explain below. The nature of the immediate species can be inferred from the energy of the respective electrons. The temporal information provided by the (core-hole-clock) experiments results from the ∼4 fs lifetime of the oxygen 1s core-hole23 considered here, and autoionization electrons are thus produced within this ultrashort timescale. Let us begin by briefly reviewing the electronic relaxation processes in core-ionized liquid water before turning to hydrogen peroxide aqueous solution. The overall oxygen 1s

INTRODUCTION Radiation-induced chemical reactions in aqueous solutions play a vital role in damaging aqueous condensed matter, including biological systems1−3 but the underlying ultrafast mechanisms and the identity of occurring reactive species are not well understood. The limited knowledge stems mainly from the poorly explored intermolecular couplings between the corelevel ionized solute and surrounding solvent molecules. Current modeling of reaction pathways in water radiation chemistry assumes that a doubly charged water molecule is the only immediate species formed upon Auger-electron decay.2,4−6 In liquid water, however, such a local on-site electronic decay competes with nonlocal processes, in which sub-10 fs relaxation involves coupling with neighboring molecules.7−9 We have recently shown that in liquid water the intermolecular energy and charge transfer is to a large extent even mediated by nuclear dynamics.10 In the present work, we explore the intermolecular relaxation processes for another hydrogenbonded aqueous system, namely hydrogen peroxide in water. Explicit time-resolved, pump−probe experiments from liquid water and from aqueous solution with X-rays are yet in their infancy, and hence most of the information on the ultrafast dynamics is revealed via experiments in the energy domain. Measurements of the ultrafast electron dynamics in aqueous solution, exploiting the so-called core-hole clock aspect,11−13 were either based on the detection of emitted X-rays or © 2014 American Chemical Society

Special Issue: John C. Hemminger Festschrift Received: May 13, 2014 Revised: June 20, 2014 Published: June 20, 2014 29142

dx.doi.org/10.1021/jp504707h | J. Phys. Chem. C 2014, 118, 29142−29150

The Journal of Physical Chemistry C

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Auger-electron spectrum from liquid water can be essentially viewed as a gas-phase water Auger spectrum with peaks being broader and shifted to higher kinetic energy.24 This largely results from electronic screening polarization and from a distribution of hydration configurations, each having a characteristic energy.16 However, there is one additional spectral feature which has no analogue in the gas-phase spectrum: a high-kinetic energy tail.7−9,24 These high-energy electrons could be interpreted as the two charges delocalized over two water units. But the situation is more complex as our analysis has demonstrated7 and as we briefly summarize here. Comparing the autoionization spectra of normal water, H2O(aq), and of heavy liquid water, D2O(aq), shows that the intensity of electron emission from the above-mentioned delocalized dicationic states is considerably larger for H2O(aq). As has been discussed in detail in ref 7, this difference is due to proton transfer upon the core ionization, implying that even during the few femtoseconds lifetime of the O 1s core hole, the proton of the core-ionized water molecule moves a sufficiently large distance toward a neighboring water molecule.7,25 The importance of nuclear dynamics for coreionized and core-exited liquid water has also been discussed in several other works typically based on detection of emitted Xrays, but the explicit electronic structure fingerprint of the occurring intermediates could not be accessed.25−31 In the evolving transient Zundel-like structure, [HO···H···H2O]+, charge is thus separated due to the moving proton and there are now two different ways to release the excess energy for this continuum of cationic intermediate. The decay processes are qualitatively the same occurring for the ground-state nuclear structures, but the outcome is quite different. One autoionization process is the Auger-electron decay,32 and another is the intermolecular coulombic decay (ICD).14,33−37 Auger decay always refers to a local (on-site) process, no matter whether it occurs for a water molecule in its ground-state structure or some proton-transferred structure. ICD, however, is a nonlocal relaxation mechanism, in which the energy released in the hole refill of the (in this case) core-ionized water molecule is used to ionize a neighbor water molecule.36 In water, ICD in fact becomes more probable for structures that evolve through proton transfer.38 Note that for the Auger branch of such proton-transfer-mediated charge separation (PTM-CS) reactive species, OH+···H3O+, are formed while the ICD branch leads to HO···H+···H2O+, as depicted in Figure SI-1 of the Supporting Information. The different species trigger different reaction channels in radiation chemistry. It is evident from the water isotope study that the heavier deuterons are less efficient for the PTM-CS process. In the present study, we exploit the isotope effect for a different hydrogen-bonded system to reveal whether the PTMCS process is a universal phenomenon or being rather specific to water, and to determine the necessary conditions for the PTM-CS to occur. We have chosen to address the above questions for aqueous hydrogen peroxide, arguably the closest analogue to water. Notably, hydrogen peroxide represents a particularly suitable candidate because its hydrated form has been well-described.39−44 Unlike for neat liquid water, in the case of H2O2(aq), the Auger electrons at the highest kinetic energies are, however, not per se indicative of the PTM-CS process. In H2O2, lower-energy dicationic states can be also populated through across-the-bond relaxation processes involving the two oxygen atoms of the molecule; the energy of these states is comparable to the one resulting from PTM-CS,10

which is an intermolecular process. We have thus to focus on the nuclear aspect of the PTM-CS process. In the previous work,10 we have already speculated, based on theoretical considerations, that proton dynamics can occur in H2O2 aqueous solution, but the experimental results were inconclusive. Experiments performed with isotopically substituted hydrogen peroxide, as are presented here, do however unequivocally identify the PTM-CS relaxation. Our experimental findings are complemented by a theoretical interpretation of the photoemission measurements. We discuss why the proton-transfer-mediated charge-transfer process takes place in hydrogen peroxide (aq). The necessary prerequisite for this type of process is the existence of a barrierless protontransfer pathway between hydrogen peroxide and a solvating water molecule (or possibly neighboring hydrogen peroxide molecule). Such pathways can be identified with ab initio methods. The existence of the proton-transfer channel does not guarantee though that the proton-transfer process actually occurs within the very short lifetime of the oxygen 1s core hole. We thus need to track the sub-10 fs dynamical evolution of the system within the first few femtoseconds following the ionization of the system. Ab initio dynamical calculations in the excited states are routinely performed for excitations and ionizations of valence electrons,45−51 but the nuclear dynamics has been rarely addressed in the core-ionized/excited states.25,26,28−31 The limited attention paid to the nuclear dynamics for core excited states is related to the short oxygen core-hole lifetime, usually assumed too short for any relevant dynamics. It ought to be mentioned that quantum effects play a rather important role in the PTM-CS process;7 the proton transfer, within the very short time available, is mostly driven by a quantum dispersion of the wave packet formed upon ionization on the core-ionized state. Here, we simulate the spreading of the wave packet using classical molecular dynamics on the ab initio potential energy surface. In this way, all degrees of freedom are taken into account. The quantum effects are accounted for by choosing the initial distribution of positions and momenta according to the Wigner distribution of the initial wave function.

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METHODS Experiment. Photoelectron and Auger-electron spectroscopy measurements were performed from a 15-μm-sized vacuum liquid-water jet18,22 at the microfocus soft-X-ray U41-PGM undulator beamline of BESSY II, Berlin. We estimate a jet velocity of ∼80 ms−1 and liquid jet temperature at the position where ionization occurs (approximately 0.1 mm downstream of the glass capillary forming the jet) to be a few degrees above 0 °C. Here we assume that the temperature is only slightly smaller than for a 10 μm water jet, in which case measurement of the velocity distribution of evaporating water molecules yields a temperature of ∼6 °C.19 An exact temperature determination has not been performed for the 15 μm water jet. 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 a normal direction with respect to the synchrotron-light polarization vector. The energy resolution of the U41 beamline was better than 200 meV at the incident photon energies used here, and the resolution of the hemispherical energy analyzer was constant with kinetic energy (about 200 meV, at 20 eV pass energy). Small X-ray focal size, 23 × 12 μm2, assures less than 29143



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obtain the Auger-electron spectrum from aqueous hydrogen peroxide. In addition to a precise energy calibration, our study thus requires the measurement of high signal-to-noise Auger spectra from the solution and from neat water under exact identical experimental conditions. This is accomplished by collecting spectra from either solution over a fairly long acquisition time of ∼20−30 min, and in addition, each measurement has been repeated several times. Since the present work aims at quantifying small differences in the Auger/autoinization spectra from H2O2aq versus D2O2aq, measurements as described above must be simultaneously performed for the two isotope solutions. We first consider the oxygen 1s photoelectron spectra. Exemplarily, we show in Figure 1 the spectra from D2O2 (aq)

5% gas-phase signal contribution to the total photoelectron signal. 3.5 M Hydrogen peroxide aqueous solutions were prepared using deionized water. For the H2O2 aqueous solution, we used H2O2 (30 wt % in H2O; Sigma) to which the corresponding amount of normal (H2O) water was added, and for the preparation of the analogous D2O2 aqueous solution, we used 30 wt % D2O2 in D2O stock solution (Deuteron). Throughout the text, the two solutions, H2O2 in H2O, and D2O2 in D2O, will be simply labeled H2O2(aq) and D2O2(aq), and we use H2O2aq and D2O2aq when referring to the individual hydrogen peroxide molecule in its respective aqueous environment. Small amount of NaCl (yielding < 0.05 M concentration) was added to compensate for charging of the liquid jet.22 Computation. Our computational strategy is as follows. We perform dynamical simulations of small H2O2(H2O)n clusters in arrangements representing typical configurations encountered in hydrated hydrogen peroxide.44,52 We then identify the coordinates important for the proton transfer, and map the potential energy surface (PES) of the core-ionized state along these coordinates. The calculations performed for the hydrogen peroxide are contrasted with analogous calculations for water. Our computations are always done in parallel for the systems with regular hydrogen and with the deuterium. Molecular dynamical simulations were performed on the core-ionized state for the H2O2(H2O), H2O2(H2O)2, and (H2O)2 clusters and for their deuterated analogues. The ground-state structures of the clusters were optimized with the MP2 method and 6-31++g** basis set for each cluster. Wigner transformation53 of the initial harmonic vibrational wave function was then performed and was used for sampling of the initial positions and momenta; see Figure SI-2 of the Supporting Information for details. The anharmonic modes with frequency below 500 cm−1 were neglected in the sampling. The dynamical runs were performed using an in-house code with a time step of 0.5 fs, and the length of each simulation was 4 fs. The standard velocity Verlet propagator was used for integration of Newton equations. The core-ionized state dynamical calculations were run using the complete active space-self consistent field (CASSCF) method, assuming only the orbitals with double occupancy in the neutral state and assuming frozen core orbitals. The specific core-ionized state was selected by enforcing the corresponding occupancies of the initial orbitals. We used the aug-cc-pVDZ basis set for hydrogen atoms and aug-cc-pCVDZ basis set for oxygen atoms. The same method was also used for mapping the potential energy surfaces. We have also analyzed the final dicationic states, using the CASSCF method with 12 electrons in 7 orbitals. The localization of the charge was investigated by Mulliken population analysis. The ab initio calculations were performed in Molpro package, version 2012.1,54 and the dynamical simulations were performed with our in-house code.

Figure 1. Oxygen 1s photoelectron spectra from 3.5 M hydrogen peroxide aqueous solution, D2O2(aq) (in red), and from heavy liquid water, D2O (in blue), both measured at 600 eV photon energy. Relative intensities are as measured.

and D2O (aq), measured at a photon energy of 600 eV. The prominent O 1s signal (in blue) at 538.2 eV binding energy (BE) arises from D2O (aq), and the shoulder in the D2O2 (aq) spectrum (red) results from ionization of D2O2aq (see Experimental for a definition); the corresponding O 1s peak position is at ∼539.2 eV BE. The value for D2O (aq) is in agreement with the reported 0.1 eV lower O 1s BE for H2O (aq) than for D2O (aq)55 and a similarly larger BE for hydrogen peroxide [with respect to H2O (aq)] has also been observed for H2O2 (aq).56 Note that the isotope effect for the photoelectron spectrum reflecting the ground-state nuclear distribution is much smaller than the effect in the respective Auger-electron spectra from water.7 In addition to these true BE differences, in Figure 1, we had to apply a +0.05 eV energy shift to the D2O2 (aq) spectrum in order to match the water O 1s peak from the neat heavy-water measurement; a similar small shift occurs for H2O2(aq) (not shown here). This energy shift mainly arises from small differences in the electrokinetic charging effects in D2O (aq) versus D2O2 (aq) and is a characteristic property of the respective solution. The observed somewhat smaller O 1s photoelectron signal from D2O2 (aq) as compared to D2O (aq) is similar to our findings for the light hydrogen peroxide in ref 10. This difference scales almost quantitatively with the twice as large rise in the D2O2aq signal than decrease of the D2O (aq) signal, reflecting the approximate 1:2 ratio expected from the higher oxygen-number density in hydrogen peroxide solutions. Figure 2 shows the Auger-electron spectra from our two solutions, D2O2 (aq) at the top, and H2O2(aq) in the center, each along with the respective water reference spectrum as explained above. Photon energy was again 600 eV. The bottom tier compares the water spectra to illustrate that the large

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RESULTS AND DISCUSSION As we have discussed in a previous work,10 the nonresonant Auger-electron spectrum of hydrogen peroxide (aq) always overlaps with the large signal from Auger electrons from water, and extraction of the small signal from the former requires an accurate quantification of spectral energy shifts due to electrokinetic charging. Although the shift is only some meV, it has a considerable effect on the differential spectra, Auger(solution) minus Auger(water) signal, from which we 29144



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Figure 2. Oxygen 1s Auger-electron spectra of 3.5 M H2O2(aq), 3.5 M D2O2(aq), and of normal and heavy neat liquid water measured at 600 eV photon energy. Top and center tiers compare hydrogen peroxide and water spectra, and the bottom tier shows the isotope effect for neat water.

Figure 3. Auger-electron spectra from the hydrogen peroxide solute, H2O2aq and D2O2aq; see main text for notation. These spectra are obtained from subtraction of the respective water Auger-electron spectrum from the solution spectrum of Figure 2. To facilitate the comparison of the two differential spectra, the D2O2aq spectrum has been shifted to 0.1 eV larger kinetic energy, which is the energy shift due to isotopic substitution as explained in the text. The smaller intensity from local Auger decay near 504 eV in the H2O2aq spectrum reflects the competition of local and nonlocal relaxation and leads to shoveling of intensity to the high-KE tail.

isotope effect in the Auger-electron spectra already exists for the neat solvent; very similar spectra have been presented in ref 7. In order to allow for a meaningful quantitative comparison between water and hydrogen peroxide (aq), the spectrum of the latter has been shifted by 0.05 eV, which is the above determined charging effect. Relative intensities of the two spectra were determined from the quantitative analysis of the X-ray absorption signal intensity from hydrogen peroxide, as described in ref 10. We find that the water spectra in Figure 2 accurately reproduce the Auger spectra of H2O (aq) and D2O (aq), and their relative differences (Figure 2, bottom) reported in ref 7. Also, the spectra in the top tier are in good agreement with our previous report for 6 molar H2O2 concentration.10 In both our previous studies,10,56 no concentration-dependent energy shifts or peak broadening was observed in the spectra (for both O 1s photoelectron and Auger spectra) for hydrogen peroxide concentrations below 6 M. Thus, the spectra obtained here, for 3.5 M concentration, are argued to contain negligible contributions associated with H2O2−H2O2 interactions. We are now set to address the actual Auger-electron spectra of light and heavy hydrogen peroxide in water. Subtraction of the water Auger spectra from the ones from hydrogen peroxide aqueous solution (of Figure 2) yields the H2O2aq and D2O2aq Auger spectra presented in Figure 3; the detailed subtraction procedure has been described in ref 10. We observe small but distinct differences; the main effect is a ∼30% larger intensity for H2O2aq in the 512−518 eV KE range and can be unequivocally attributed to proton-transfer dynamics followed by autoionization (PTM-CS). The same kineticenergy region was already speculated to contain signal contributions due to intramolecular charge delocalization.10 Note that the KEs of the Auger electrons detected here are close to 600 eV KE, corresponding to probing photoionization events from the surface up to 45−50 Å into the bulk of solution.57 In our experience, spectra collected under these conditions provide a reliable measure of the average bulk solution behavior.58 The dynamical processes reported here might be different though at the solution interface but cannot be investigated at the large KE of the oxygen 1s Auger electrons.

In the following, the dynamics of the proton transfer upon the core ionization will be analyzed for several model clusters by means of theoretical calculations. Optimized ground-state structures are shown as insets in Figure 4. The three cases discussed here represent the smallest clusters but highlighting the different principle structural arrangements. The H2O2··· H2O dimer complex (Figure 4B inset) and the symmetric structure of the H2O2(H2O)2 trimer (Figure 4D inset), with water on either side of H2O2, do not exhibit the typical features of the hydrogen bond. The water unit instead forms a bridge, thus acting partially both as a hydrogen-bond donor and hydrogen-bond acceptor. The H2O2(H2O)2 asymmetric structure (Figure 4B inset) with both water units on one side of hydrogen peroxide (global minimum on the potential energy surface42) already leads to a formation of proper hydrogen bonds. One water unit acts as the hydrogen bond donor, the other as the hydrogen-bond acceptor. For comparison, Figure 4A also displays the structure of the water dimer, where the hydrogen bond is clearly formed. We anticipate that structures which form true hydrogen bonds will strongly support the proton transfer along a linear O−H···O arrangement. Figure 4 shows the hydrogen densities projected onto the O−H (or O−D) axis of hydrogen peroxide (or water) for two time instants, t = 0 fs (initial state; solid lines) and t = 4 fs (corresponding approximately to the mean lifetime of the oxygen core hole; dashed lines). Let us first focus on the case of water dimer (Figure 4A), where the PTM-CS process has been clearly identified.7 Even before the ionization, a very small difference in the hydrogen density can be observed for regular versus heavy water dimers. This effect reflects the slightly different ground-state vibrational wave functions. The difference significantly increases within the first 4 fs; the first few femtoseconds indeed suffice for a considerable fraction of molecules to almost complete the proton transfer. When we replace one of the water units with hydrogen peroxide (see Figure 4B), only limited proton transfer is observed. The hydrogen distributions along the O−H bond change only slightly, and yet this small effect is less pronounced for D2O2. Apparently, the geometry arrangement in the H2O2···H2O complex is not supportive of the hydrogen transfer. Similar 29145



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Figure 4. Distributions of O−H distance indicating the proton transfer from H2O2 to H2O in the H2O2(H2O)n complexes and for water dimer. Red lines correspond to deuterated analogues. The distributions at time t = 0 fs are shown as full lines, and distributions at time 4 fs are shown as dashed lines. Dynamical simulations for the following structures are analyzed: (A) water dimer (H2O)2, (B) hydrogen peroxide···water dimer, and (C) asymmetrical H2O2(H2O)2, and (D) symmetrical H2O2(H2O)2 complex. The insets depict the respective ground-state structure of each complex optimized at the MP2/6-31++g** level of theory. Relevant distances and angles are shown as well.

Figure 5. Two-dimensional cuts through the PES for the H2O2···H2O complex. (A) Scan along the hydrogen transfer (O−H distance) coordinate versus the hydrogen bond angle (O−H···O) coordinate. See the structure in Figure 4B for the depiction of the angle. The O···O distance between water molecule and hydrogen peroxide was set to 2.65 Å (i.e., the most probable distance in aqueous solution).44 (B) Scan along the hydrogentransfer coordinate versus the O···O distance coordinate. The hydrogen-bond angle was fixed at 180 deg to represent the structure most probably found in peroxide solution.

H2O complex as a model. As pointed out above, this complex does not promote the proton transfer in its ground-state equilibrium geometry. We will show here that this is due to the unfavorable arrangement of the hydrogen bond. For this, we consider the O−H coordinate involved in the proton transfer. The second coordinate we take into account is the OH2O··· HH2O2−OH2O2 hydrogen bond angle. Figure 5A shows the 2dimensional scan through the PES in these two coordinates. It

observation holds even for the symmetrically hydrated hydrogen peroxide (Figure 4D). The situation is quite different for the asymmetrically hydrated H2O2(H2O)2 complex (see Figure 4C). Here, the proton transfer is pronounced, and as for the water dimer, the rate of the transfer is much faster for regular hydrogen in comparison with the heavy hydrogen atom. We have further investigated the geometry arrangements which would facilitate the PTM-CS process, using the H2O2··· 29146



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Figure 6. Electronic energies of the doubly ionized H2O2···H2O complex as a function of the proton-transfer coordinate from hydrogen peroxide to water. The adiabatic states corresponding to charge being localized on H2O2 are depicted by red dots, and the green dot indicates the charge localized on the water molecule. Note that the character of the states change along the proton-transfer coordinate. The initial structure was taken from the asymmetric H2O2(H2O)2 complex with the donor-water molecule deleted. The energies were shifted with respect to the second ionization energy at the initial geometry calculated at the CCSD(T)/aug-cc-pVTZ level of theory.

the PTM-CS process. Figure 6 shows the potential energy curves of the doubly charged H2O2···H2O complex. The Auger spectrum should be dominated by a local contribution in the ground-state geometry.38 Mulliken population analysis of the electronic wave function reveals that the first local doubly charged state of H2O2 in Figure 6 is the sixth state, and the doubly charged state of H2O is the 20th state. Qualitatively, the observed high-energy tail in the Auger spectrum should correspond to the proton transfer from the hydrogen peroxide molecule; with the transfer of the proton, the energy of this state is lowered, and this decrease in energy is smaller than for water. The Coulombic energy gain by the charge delocalization is clearly smaller than in water, where the double charge is confined on a single atom. From the results of CASSCF calculations, a 5 eV Coulombic energy gain for the water dimer was estimated for O−H distance of 1.4 Å,7 while the similar calculations for hydrogen peroxide (Figure 6) suggest a ∼2.5 eV stabilization.

is clearly seen that the O−H prolongation leads to the hydrogen-transferred product only in a barrier-free process, when the OH2O···HH2O2−OH2O2 bond is becoming linear. We also explored how the PTM process depends on the distance, O···O, between the oxygen atom of the hydrogen peroxide and the oxygen atom of the solvating water molecule. As can be seen in Figure 5B, a large barrier gradually appears for longer O···O distances. Two conditions are thus to be met in order to observe proton transfer: a linear O−H···O arrangement and not too large O···O distance. In liquid water, the H···O distance corresponding to the first solvation layer is typically at a value of 2.65 Å, and the hydrogen bond is linear.44 Hydrogen peroxide was found in several studies44,52 to be a better hydrogen donor than water, the HOOH···OH2 distance is on average some 0.04 Å shorter than in water,59 and the hydrogen bond stays essentially linear.52 The proton transfer from hydrogen peroxide to water should thus be possible. Our experimental results seem to indicate that the isotope effect is less pronounced for hydrogen peroxide than for water. Interestingly, the calculated proton transfer proceeds with almost equal rate, and the difference between H and D is almost the same for both the water dimer and the hydrogen peroxide···water complex, in a configuration with a proper hydrogen bond (as in Figure 4, panels A and C). One possible reason for the discrepancy is that the hydrogen-bond arrangement is on average less favorable for the PTM-CS process in the case of H2O2 than for water. Hydrogen peroxide is hydrated in a more complicated way than water, and the number of hydrogen bonds per single oxygen atom is smaller.44,52 The smaller PTM-CS signal for hydrogen peroxide may also result from slightly smaller oxygen 1s core-hole lifetime of hydrogen peroxide as a consequence of across the O−O bond relaxation. Finally, the smaller isotope effect on the high-energy tail of the Auger spectrum might be due to the nature of the final state dications. We have therefore analyzed the character of the doubly charged final states formed within

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CONCLUSIONS Isotopically resolved Auger-spectroscopy from H2O2 (aq) and D2O2 (aq) unequivocally identifies the proton-transfer process following the oxygen core-level ionization of the hydrogen peroxide molecule. The nuclear dynamics occurring for hydrogen peroxide within the oxygen core-hole lifetime is confirmed by ab initio molecular dynamics simulations. The socalled charge-transfer mediated charge separation (PTM-CS) is thus demonstrated to be not unique to liquid water but is likely to be a general relaxation channel in hydrogen-bonded systems. Unlike in water, for H2O2 (aq) the dicationic final states have very similar energies as the states that correspond to delocalization of the two charges across the H2O2 molecule. The present isotope experiment does distinguish between these two situations. Our calculations have identified necessary conditions for the PTM-CS to take place (i.e., linear arrangement of the hydrogen 29147



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bonds and sufficiently short O···O distances between the hydrogen bond donor and hydrogen bond acceptor). The sensitivity of the PTM-CS process on the details of the hydrogen-bond arrangement could in principle be used for exploration of the liquid structure. The PTM-CS process and its detailed quantitative analysis might, for example, contribute to an ongoing debate on whether liquid water is indeed essentially arranged tetrahedrically as is the present-day canonical view60−62 or whether a dissenting picture of the liquid water structure28,63 is reasonable. The manifestation of the PTM-CS process in the Augerelectron spectrum is less pronounced for hydrogen peroxide than it is for neat water. We argue that this is perhaps due to a weaker dependence of the final dicationic energies on the proton transfer for the solvated hydrogen peroxide. The mere fact that PTM-CS processes occur in hydrogen peroxide aqueous solutions has considerable consequences for future modeling of the processes and reactions encountered in radiation chemistry. Hydrogen peroxide is one of the irradiation products, and it is therefore important to know how it is transformed upon further irradiation.4 Even though the present study does not reveal the details of the autoionization process, which could be by Auger-PTM-CS or by ICD-PTM-CS, we can expect a series of so far unexpected (reactive) species to form. The Auger channel would produce O2H+···H3O+, and the ICD channel leads to HO2···H+···H2O+, and each of these species will involve in specific chemical reactions.

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ASSOCIATED CONTENT

S Supporting Information *

Schematic of two autoionization channels of the transient proton-transferred anionic structure; role of initial conditions; comparison of hydrogen atom distributions along the O−H distance indicating the proton transfer from H2O2 to H2O in the asymmetric H2O2(H2O)2 complex; proton transfer between water and hydrogen peroxide; and potential energy surfaces of all core ionized states along the respective O−H coordinate in the H2O2(H2O)2 asymmetric complex. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.S. thanks the support of the Grant agency of the Czech Republic via Grant 13-34168S. D.H. acknowledges the financial support from specific university research (MSMT No 20/ 2014); he is also a student of International Max Planck Research School ‘‘Dynamical Processes in Atoms, Molecules and Solids’’. E.F.A. thanks support through the European Research Council Grant 279344, and B.W. and I.U. acknowledge the support from the Deutsche Forschungsgemeinschaft (DFG) via the DFG Research Unit FOR 1789.

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