Relaxation Processes in Aqueous Systems upon X-ray Ionization

Perspective pubs.acs.org/JPCL

Relaxation Processes in Aqueous Systems upon X‑ray Ionization: Entanglement of Electronic and Nuclear Dynamics Petr Slavíček,*,† Nikolai V. Kryzhevoi,*,‡ Emad F. Aziz,*,§,∥ and Bernd Winter*,§ †

Department of Physical Chemistry, University of Chemistry and Technology, Technická 5, 16628 Prague, Czech Republic Theoretische Chemie, Physikalisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 229, D-69120 Heidelberg, Germany § Helmholtz-Zentrum Berlin für Materialien und Energie, Methods for Material Development, Albert-Einstein-Straße 15, D-12489 Berlin, Germany ∥ Department of Physics, Freie Universität Berlin, Arnimallee 14, D-14159 Berlin, Germany ‡

ABSTRACT: The knowledge of primary processes following the interaction of high-energy radiation with molecules in liquid phase is rather limited. In the present Perspective, we report on a newly discovered type of relaxation process involving simultaneous autoionization and proton transfer between adjacent molecules, so-called proton transfer mediated charge separation (PTM-CS) process. Within PTM-CS, transients with a half-transferred proton are formed within a few femtoseconds after the core-level ionization event. Subsequent nonradiative decay of the highly nonequilibrium transients leads to a series of reactive species, which have not been considered in any high-energy radiation process in water. Nonlocal electronic decay processes are surprisingly accelerated upon proton dynamics. Such strong coupling of electronic and nuclear dynamics is a general phenomenon for hydrogen-bonded systems, however, its probability correlates strongly with hydration geometry. We suggest that the newly observed processes will impact future high-energy radiation-chemistry-relevant modeling, and we envision application of autoionization spectroscopy for identification of solution structure details.

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unknown. The reason for our ignorance is clear: the primary processes occur on a femtosecond or subfemtosecond time scale,6−8 and the detection of the transient species in aqueous phase was until recently virtually impossible. By now, the ultrafast processes following the interaction of X-rays with matter can be tracked directly in real time, e.g., using free electron laser technology. Alternatively, the nature of these processes can be aptly explored in the energy domain by photoelectron spectroscopy, detecting direct photoelectrons or secondary electrons. From the measured distribution of the photoelectron kinetic energies one infers electron binding energies corresponding to the energies of the occupied orbitals, which provides basic information on the electronic structure of matter.4 Photoemission spectroscopy of secondary electrons reveals information on the radiation-induced relaxation processes and identifies the short-lived transient species formed upon the ionization/excitation. In this Perspective, we focus on both these aspects of photoemission spectroscopy. The experimental exploration of electronic processes in the liquid-phase was greatly spurred by the liquid-microjet technique.9,10 Only after its introduction, high-precision photoemission spectra could be recorded for highly volatile liquid-phase systems, also paving the way to the exploration of ultrafast relaxation processes in aqueous media.11 Briefly, the

aturally occurring high-energy radiation represents a decisive factor in the chemical evolution of the universe1 and is at the same time a major cause of radiation damage of biomolecules.2 Artificial sources of X-rays have become an integral part of our lives with applications ranging from medicine to material development. For example, ionizing radiation can be used for material transformation,3 or it is applied in radio-oncology, which despite the great advancement in pharmacology, still remains one of the most important approaches for cancer treatment. Finally, X-rays represent an indispensable component in atomic and molecular spectroscopy, providing detailed insight into the electronic and geometric structure of matter and revealing the nature of bonding in molecules and interactions in materials.4

More than 120 years after the discovery of X-rays, our knowledge of the interaction of X-rays with matter remains surprisingly humble. More than 120 years after the discovery of X-rays, our knowledge of the interaction of X-rays with matter remains surprisingly humble. The appearance of the symbol ⇝ in radiation-chemistry monographs5 is symptomatic. This symbol denotes “under the effect of ionizing radiation, irrespective of the mechanism”, indicating that the molecular details are © 2015 American Chemical Society

Received: November 30, 2015 Accepted: December 29, 2015 Published: December 29, 2015 234

DOI: 10.1021/acs.jpclett.5b02665 J. Phys. Chem. Lett. 2016, 7, 234−243

The Journal of Physical Chemistry Letters

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electron decay (Figure 1A), the electron emitted upon the corehole refill originates from the same molecule where the core ionization happened. This local relaxation channel yields a doubly ionized system with two outer-valence holes residing on a single monomer; we denote the respective two-hole final states as 2h. In the presence of weakly interacting neighbors, electronic relaxation may create two charges distributed between two different molecular units. To accentuate the delocalized nature of these two-hole final states, we denote them as 1h1h. One of the possible electronic relaxation processes leading to delocalized states is aforementioned ICD, which is sketched in Figure 1B. Here, the initially created vacancy is refilled by an electron from the same molecule releasing energy that is transferred to a neighboring unit. This latter unit emits an electron known as the ICD electron. Originally, ICD was theoretically predicted for inner-valence ionization of clusters where the Auger channel is energetically closed. The first evidence of ICD in aqueous solution was found for hydroxide anion dissolved in water;24 it is also the first study to report ICD upon core-hole excitation. In subsequent works, the important role of ICD in the relaxation of core vacancies in hydrogen-bonded systems has been addressed experimentally and theoretically for several other solutes.20,24−28 The other yet more complicated nonlocal process, ETMD (Figure 1C), was also originally predicted to occur after inner-valence ionization.23 But as in the case of ICD, ETMD may as well follow core ionization.20,29 Contrary to the Auger decay and ICD, in ETMD the (core) hole is refilled by a valence electron from a neighboring molecule, and the secondary electron, the ETMD electron, is ejected from a third neighbor. Again, two singly charged ions are formed, and the final states are also of 1h1h type. However, none of the charges is localized on the originally ionized molecule. More details on ICD and ETMD can be found in two review articles.30,31

liquid-jet technique enables stabilization of a free-flowing liquid water surface in vacuum. Due to the small diameter, watervapor density quickly reduces with distance from the jet, which results in a large increase of the electron mean free path, large enough that electrons ejected from the liquid phase can reach an electron-energy detector, at a few millimeters distance from the jet, without losing energy in collisions. Experiments reported here use soft X-rays from the synchrotron-radiation facility BESSY II, Berlin, for ionization of the liquid jet. The jet velocity is approximately 80 m·s−1, and the temperature is typically 4−6 °C.9,10 Solute concentrations are up to a few moles per cubic decimeter in order to obtain sufficiently large electron signals. We discuss aqueous systems, where solutes can be core-ionized both without and with core-ionization of the solvent water molecules. In the latter case, the solute molecules contain oxygen, and their core-level binding energies overlap with the distribution of the O 1s binding energies of the solvent. We next briefly review the relaxation processes that occur upon core-electron ionization. Gas-phase molecules with a core hole are in a highly electronically excited state, and the excess energy is released by emission of either a secondary electron (Auger-type processes, major channel) or an X-ray photon (Xray fluorescence, minor channel).4 In the former case, a core hole is refilled by a valence electron, and another valence electron is ejected. Such a process occurs on the ultrafast time scale and therefore is considered to be (i) purely electronic and (ii) local. In particular, it is assumed that the geometry does not change during the Auger decay. None of the above assumptions, however, is fully justified in condensed-phase systems, characterized by weak bonding interactions between the constituting monomers. Here, nuclear dynamics can contribute to the relaxation of core-ionized states as has been discussed for liquid water and several aqueous solutions.12−21 Furthermore, in the past two decades various nonlocal electronic decay processes have been identified, for example, intermolecular Coulombic decay (ICD)22 and electron transfer mediated decay (ETMD)23 where the secondary electron is emitted from a monomer neighboring the initially ionized one. All these processes seem to be more important than previously thought, particularly because they are found to play a considerable role in aqueous solutions as we discuss here. Figure 1 illustrates the local and nonlocal electronic relaxation processes in core-ionized molecules which have neighbors (excitation will not be discussed). In the Auger-

Here, the aforementioned electronic relaxation, local and nonlocal, is coupled with nuclear dynamics. We have recently discovered that in core-ionized liquid water and other hydrogen-bonded aqueous systems a unique class of relaxation processes occurs.18−21 Here, the aforementioned electronic relaxation, local and nonlocal, is coupled with nuclear dynamics. This means that the secondary ionization (autoionization) takes place also from transient molecular structures evolving within the few-femtosecond lifetime of the core hole; for the oxygen 1s level of water, the lifetime is only 4 fs. The nuclear dynamics leads to the formation of a whole new class of primary reactive molecular species in aqueous solution. It is the goal of this Perspective article to review this new research field, present the existing data, and discuss the impact these ultrafast relaxation processes may have in the wider fields of physical chemistry and biology. We start by presenting the coupled electron and nuclear dynamics in liquid water and then discuss the importance of such processes for other hydrogen-bonded aqueous solutes, namely hydrogen peroxide, as well as ammonia and glycine. These systems exhibit very different hydration patterns, allowing for the identification of important mechanistic details of the relaxation processes.

Figure 1. Schematic energy diagrams depicting the Auger decay (A), ICD (B), and ETMD (C) processes generated by core ionization. Starting point is a molecule with a core-level hole (1). Different holerefill routes are indicated by a black arrow. The resulting electron− hole in the valence level of either molecule (1), (2), or (3), and the subsequently emitted secondary electron are shown by white and yellow circles, respectively. The kinetic energy of the latter is denoted eKE, and the energy of the valence and core-level states is eBE. The figure has been adapted from ref 20. 235



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the ground-state structure. The competing nonlocal ICD and ETMD processes in liquid water leading to 1h1h final states are shown in Figures 2A (left column, center, and left column, bottom, respectiely). In the ICD channel, a pair of singly (valence) ionized water molecules, H2O+···H2O+, is formed, and ETMD is likely to produce two water cations bridged by a neutral water molecule, H2O+···H2O···H2O+. The core-ionized highly nonequilibrium state, H2O+*, however, also promotes the aforementioned proton dynamics, wherein a proton starts to move toward a neighboring water molecule. The transient structure which evolves within the O 1s lifetime resembles a Zundel-like cation with a proton bridging two oxygen atoms, as illustrated in Figure 2A (right column). Different electronic decays and their outcomes resulting from what we denote proton-transfer mediated charge separation processes (PTMCS) are depicted. The top box shows Auger decay of the proton-transferred molecular structure; we refer to a PTMAuger process. The dicationic structure HO +···H+···OH2 formed in this process will turn into HO+···H3O+. Analogous illustrations for the PTM-ICD and PTM-ETMD processes are given in the center and bottom boxes, respectively, of the right column of Figure 2A. The immediate species created by these processes (see the bottom of each box) differ noticeably from those formed upon autoionization of the ground-state geometry. Before presenting the experimental evidence of the PTMautoionization processes we discuss how they should be

The nuclear dynamics leads to the formation of a whole new class of primary reactive molecular species in aqueous solution. Water is an archetype for a strongly associated hydrogenbonding liquid, and it is arguably the most important species from the perspective of radiation chemistry. A large fraction of the radiation damage of biomolecules is mediated by water, that is, biomolecules are attacked by particles formed upon water radiolysis.2,32,33 In living cells such indirect effects are argued to be responsible for 50%32 or up to 66%33 of the radiationinduced double-strand breaks in DNA, with the remaining fractions resulting from direct effects, where energy is directly deposited into DNA or water molecules bound tightly to it. The relaxation processes in liquid water thus deserve special attention. Ionization and electronic decay of the O 1s core level of a water molecule can be described as hν + H2O → H2O+* + ephot → H2O2+ + ephot + eAuger. Here, the symbol * refers to the water cation with an oxygen 1s hole. The right-hand side of the equation represents the O 1s Auger-electron decay leading to a localized 2h final state (see Figure 1A). This relaxation process is illustrated in Figure 2A (left column, top) for liquid water and is depicted here for the case of a purely electronic decay, that is, occurring in a fixed nuclear geometry corresponding to

Figure 2. (A) Auger decay, ICD, and ETMD processes in liquid water after O 1s core-level ionization, illustrated for a water pentamer. The left (green) column shows the processes in the ground-state water structure, and the right (orange) column shows the processes for a proton-transferred Zundel-type cationic structure. The molecules participating in the decay processes are enclosed in brackets. The decay outcomes are shown below each structure. ET denotes energy transfer. (B) Schematic representation of the electronic and nuclear relaxations in liquid water. Shown are the sketched potential energy curves of the ground state (H2O(aq), black), core-ionized state (H2O+(aq), orange), and final states (H2O2+(aq), green). Photoionization projects (thick orange arrow) the ground-state wave packets for normal and deuterated water (solid blue and red lines, respectively) onto the intermediate core-ionized-state surface, which is repulsive along the proton-transfer coordinate. Within 4 fs, the wave packets evolve, all the more for normal water than deuterated (dotted blue and red lines, respectively). Within this time interval, autoionization occurs from continuously varying structures (thin arrows, orange to green), populating local 2h states and charge-separated 1h1h states. For clarity, only one of the nonlocal electronic relaxation processes, ICD, is included in this figure. The different types of 1h1h states, (1h1h)ICD, (1h1h)PTM‑ICD, and (1h1h)PTM‑Auger, are explained in the text. Panel A has been adapted from ref 20 and panel B from ref 18. 236



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Figure 3. Photoemission spectra from light and heavy water measured at 600 eV photon energy. Auger, O 1s, and valence electron spectra are seen at their characteristic energies; note that the valence spectra shown in the inset were measured at 200 eV photon energy. The O 1s electron binding energy is approximately 538 eV,18 and the Auger signal has a maximum near 503 eV kinetic energy. As indicated in the insets, the photoelectron spectra of heavy water occur at binding energies which are larger by up to 200 meV than for regular water. For the detection geometry used (electron detection angle normal to light polarization vector), the O 1s emission is strongly suppressed with respect to the Auger-electron signal intensity. The reported Auger electron yield is approximately 88% per core-level ionized oxygen atom.38

potential curve (solid green) that the energy of the final state resulting from PTM-Auger decay (denoted 1h1hPTM‑Auger in the figure) decreases quickly along the O−H coordinate. Thus, the PTM-Auger mechanism should yield increased electron signal in the high-kinetic energy tail, and the effect should be more conspicuous for light than for heavy water. Also, the PTM-ICD decay would lead to an intensity enhancement at the highenergy tail of the spectrum. This process is expected to be more prominent for the core-ionized Zundel-like structure transients than for the ground-state geometry as the electronic clouds considerably overlap due to the bridging proton. Below, we support the qualitative picture of Figure 2B by theory, and we also include the PTM-ETMD channel. Let us start with presenting complete photoemission spectra of normal and heavy liquid water, H2O(aq) and D2O(aq) in Figure 3, to illustrate the two major origins of emitted electrons occurring when exposing liquid water to 600 eV photons. The largest contribution is due to direct photoelectrons, near 538 eV electron binding energy, arising from oxygen 1s ionization, whereas contributions due to valence ionization are much smaller. Because O 1s ionization is inevitably accompanied by Auger decay, autoionization which gives rise to electron emission near 500 eV kinetic energy, dominates the X-ray induced processes. Isotope effects in the photoelectron spectra (see the two inset figures) are very small. Figure 4b shows the Auger spectral region for H2O(aq) and D2O(aq), measured at higher resolution than in Figure 3, again using 600 eV photon energy. For comparison, we also show a spectrum from the water jet slightly moved out of the focus of the X-ray beam (Figure 4a), in which case mostly gas-phase water is ionized. This spectrum has been aligned for a 4.5 eV gas-to-liquid-phase energy shift arising from liquid water’s electronic polarization.11 In this way, the peaks originating from the main Auger transition (K-1b11b1) line up for the two phases. Two clear observations can be made immediately. First, the liquid-phase spectrum has an extra signal in the highkinetic-energy range 505−515 eV (yellow-shaded area). This is an unequivocal and expected signature of the 1h1h states that is

qualitatively identified in a photoemission experiment. For this we consider the proton motion in the O 1s core-ionized state in liquid water. The proton motion is expressed by the temporal evolution of the respective wave packet (shape and position) on the core-ionized potential along the O−H proton-transfer coordinate, as inferred from wave packet calculations. This potential-energy curve lies at approximately 538 eV above the ground state of the neutral water as schematically shown in orange at the top of Figure 2B (the corresponding potentials obtained from electronic-structure calculations can be found in ref18). The figure also shows the potential-energy curves of the local, 2h (solid green curve), and nonlocal, 1h1h (dashed green curve), final states. Importantly, the 1h1h states have considerably smaller energies than the 2h states due to reduced Coulomb interaction between two positive charges residing on different molecular units. As a result, the secondary electrons associated with the former states have higher kinetic energies, and the high-kinetic energy side of the Auger spectra is thus the unique signature of the 1h1h final states. The electron signal in this energy range is of prime interest for our study. In order to understand the effects of proton dynamics we need to consider the actual nuclear wave packets and the shapes of the potential energy curves for the core ionized state, as well as for the 2h and the 1h1h final states for larger O−H distances (orange-shaded area of Figure 2B). Several important observations can be made. First, the cationic potential is downhill, implying that the proton transfer is energetically allowed. This is not the case for gas-phase water, which does not dissociate upon O 1s ionization. Second, the rate of the proton transfer can be modified by isotope substitution between hydrogen and deuterium. As illustrated in Figure 2B, and found in our calculations,18 the wave packet dispersion within the 4 fs core-hole lifetime of oxygen is much larger for H2O(aq) (dashed blue) than for D2O(aq) (dashed red), well intruding into a neighbor water molecule in the former case. We can thus exploit the isotope effect, with lighter mass moving further within the ultrashort observation window, to identify PTM-CS processes. We see from the steep slope of the 2h237



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within the 4 fs O 1s core-hole lifetime. Note that contributions of all geometries along the proton-transfer coordinate were taken into account by averaging over the shape of the wave packet propagating on the core-ionized potential. The computations well reproduce the isotope effect seen in the experiment, both in the central spectral region and in the high kinetic-energy shoulder. Because in liquid water many more water molecules may participate in nonlocal decay processes compared to the finite-size model pentamer considered in the calculations, the shoulders in the experimental spectra are more pronounced. We also note that in liquid water, the very flexible hydrogen bonds give rise to a large number of geometric configurations of slightly different energies. The experimental spectral shapes are thus broader than the theoretical spectra which are associated with a single representative hydration structure only. The calculations also allow us to analyze the nature of the electronic relaxation processes for different nuclear arrangements. In the inset of Figure 4, we show the computed Auger spectra for the ground-state geometry (tier (d)), for the Zundel-like structure (tier (e)) and for a water−hydronium complex (tier (f)) characterized by different positions of the proton (evolving as a function of time) between the coreionized and a neighboring water molecule. We observe that the nonlocal ICD and ETMD processes are minor channels for the ground-state molecular structure, and Auger decay is the prevailing relaxation pathway here. However, the former processes quickly gain efficiency upon proton transfer, and in the Zundel-like intermediate the local and nonlocal relaxation pathways are already equally important. Another surprising finding is the high efficiency of ETMD, showing that the process strongly competes with ICD and even with the local Auger decay. It is also worthwhile to mention that the large contribution of nonlocal decay processes significantly modifies the molecular products of the electronic and nuclear relaxations. Together with dicationic water monomers, a plethora of various highly reactive neutral and cationic radicals forms, potentially initiating biodamage. Using the relative populations of different final states computed in ref 20 (see Figure 5 therein), we can compute the relative yields of various electronic decay products. We assume that water dications are mainly produced by Auger decay, pairs of water cations are produced by ICD and ETMD, and the so-called PTM fragments (OH+, OH•, H3O+, etc.) by all PTM-autoionization processes which occur for geometries with the proton transfer coordinate larger than 1.4 Å. Water dications are found to be the major products at the initial electronic relaxation, which is consistent with tier (d) of the inset Figure 4. The production of these species decreases at the expense of water cations and PTM fragments when time evolves. When all the autoionization processes are completed the relative accumulated yields amount to 51% for H2O2+ and 45% for H2O++ H2O+, wherein 20% are due to ICD and 25% due to ETMD. The remaining fraction is due to PTM fragments. Using these numbers as the efficiencies of the corresponding processes, and the Auger lifetime of 4 fs, we can estimate for the water pentamer, by applying the core− hole clock method,26 the ICD and ETMD lifetimes of 10 and 8 fs, respectively. A question that arises is whether the PTM-CS process is universal for hydrogen-bonded systems and specifically for aqueous solutions. And if so, can we use liquid-phase Auger spectroscopy for exploring the liquid-state structure of systems with hydrogen bonds? To find out, we studied three small

Figure 4. Main figure: Experimental (tier (b)) and computed (tier (c)) oxygen 1s Auger electron spectra from light (in blue) and heavy (red) liquid water. The experimental spectra were measured at 600 eV photon energy. Simulations were performed for a water pentamer. The theoretical spectra are shifted to larger energies by 3.4 eV to account for long-range polarization effects that are apparently missing in the finite size pentameric system. Tier (a) is the experimental Auger spectrum of water vapor exhibiting no high-kinetic energy shoulder associated with the 1h1h states (yellow-shaded region). The spectrum is shifted by the gas−liquid phase shift, as explained in the text. Inset: Simulated Auger-electron spectra (black curves) for three explicit geometries of the water pentamer (as depicted), representing structure changes upon proton dynamics. Results are shown for the protontransfer coordinates 0.95 Å (ground-state geometry, tier (d)), 1.40 Å (Zundel-like structure, tier (e)), and 1.85 Å (water−hydronium complex, tier (f)). The areas under the gray (shaded area), red, and blue curves reflect the contributions of the Auger, ICD, and ETMD processes, respectively, to the total spectral intensity. The figure has been adapted from ref 20.

absent in the spectrum of gas-phase water where no charge delocalization occurs. The second finding is a noticeably larger population of the 1h1h states for light water due to the faster nuclear dynamics than for heavy water (see Figure 2B). In the absence of the PTM-CS processes such a large isotope effect does not occur because the energy differences for the groundstate electronic structure of light and heavy water is in the 100 meV range (compare Figure 3), that is, much smaller than observed in Figure 4b. The experiment is not able to resolve all details as to the exact nature of the processes outlined above because the spectral domains of PTM-Auger decay, PTM-ICD, and PTMETMD processes strongly overlap. The nature and relative importance of the different relaxation processes can however be evaluated from computations. The simulated electronic decay spectra shown in Figure 4 tier (c) and in tiers (d) through (f) of the inset figure were obtained by calculating the energies and relative populations of all final dicationic states for model water pentamers (H2O)5 and (D2O)5 (electronic decay of the central water molecule was considered only).20 The spectrum shown in tier (c) represents an integration of signals in the time span between 0 and 9.6 fs, which well covers the dynamics occurring 238



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points for H2O(aq) and H2O2(aq), but not for NH3(aq). Thus, aqueous ammonia tends to form on average considerably weaker intermolecular hydrogen bonds, and PTM-CS is anticipated to be less likely here. As for liquid water, the PTM-CS mechanism is inferred from isotope-substitution experiments. That is, we measure the O 1s Auger-electron spectra from H2O2 in light liquid water, denoted H2O2(aqH), and from D2O2 in heavy liquid water, denoted D2O2(aqD). Results are presented in Figure 6A. Both spectra were obtained from 3.5 M solutions using 600 eV photon energy, the same energy we applied in the neat-water study. Because the O 1s electron binding energy of H2O2(aq) is only 1 eV smaller19,34 than for H2O(aq) the autoionization spectra from water and hydrogen peroxide strongly overlap and the electron signal due to the solute must be carefully extracted. Comparison of the spectra for the two isotopes reveals a somewhat larger peak for H2O2(aqH) near 515 eV kinetic energy. This peak is entirely attributed to the 1h1h states resulting from PTM-autoionization of H2O2(aq).19 Another peak, near 510 eV kinetic energy, is also due to 1h1h states but arises from the delocalization of two valence holes within the hydrogen peroxide molecule.34 Hydrogen peroxide is expected to be a stronger or at least an equally good hydrogen-bond donor than water (see Figure 5). Also, the proton-transfer processes in water and hydrogen peroxide proceed similarly. This is illustrated in Figure 7, showing simulated proton-transfer dynamics for the systems discussed here. Frame A of Figure 7 depicts the results for the water dimers (H2O)2 and (D2O)2, and Frame B presents analogous data for the smallest hydrogen-peroxide clusters allowing for proton transfer, H2O2·(H2O)2 and D2O2·(D2O)2.19 Densities along the proton-transfer coordinate are shown for time zero (ground-state structure) and for 4 fs after the O 1s ionization. The wave packet dispersions are found to be very similar for hydrogen peroxide and water. Why then is the isotope effect in the experimental Auger spectrum of H2O2(aq) considerably smaller than for liquid water? We largely attribute this behavior to shallower dicationic final states along the proton-transfer coordinate in aqueous hydrogen peroxide.19 The charge separation via the proton transfer causes a smaller energy change for the H2O2 molecule compared to H2O. In the former case, the proton interacts electrostatically with a positive charge distribution delocalized across the O−O bond, whereas in the latter case, the charge distribution is essentially confined on a single oxygen atom. This is quantitatively reflected in our calculated energies,34 which are also in good agreement with above assignment of the high-kinetic energy contributions due to 2h and 1h1h states. The analysis of the hydrogen peroxide spectra thus shows that the relation between the intermolecular hydrogen-bond strength and the PTM-CS signal is not trivial. Nonetheless, the fact that proton transfer is observed, hints at PTM-CS being a rather general process in the condensed aqueous phase. For ammonia in water, we would expect a much smaller PTM-CS signal than for neat water or hydrogen peroxide based on the strength of intermolecular hydrogen bonding (see Figure 5). Indeed, the suitable arrangement for the efficient proton transfer is reached for only a small fraction of geometric configurations. On the other hand, the core-hole lifetime is almost twice as large for nitrogen compared to oxygen, and there is thus a longer time for observing the hydrogen transfer. It is therefore not clear what should be expected for the efficiency of the PTM-CS process. At first glance, the PTM-CS

aqueous molecular solutes of distinctly different hydration configurations. The first solute is hydrogen peroxide. It was chosen because H2O2 represents a more complex analogue of water, and it is supposed to be an equally good hydrogen-bond donor as water. Yet hydrogen peroxide’s internal molecular structure leads to more complicated hydrogen-bond patterns. The ammonia (isoelectronic with water), which is the second solute studied, on the other hand, interacts more weakly with water. Ammonia is also an important candidate when exploring coupled electronic and nuclear relaxation processes in more complex biomolecules such as proteins containing an amino group. To highlight the latter aspect, we also discuss the simplest amino acid glycine. For each molecule, our focus is on the PTM-CS processes along the X−H coordinate in X−H··· OH2 complexes, where X is the core-ionized O or N atom. On the basis of the findings for liquid water, our expectation is that nuclear dynamics is more probable the stronger the intermolecular hydrogen bonds are. To quantify hydrogenbond strengths, we consider Figure 5. Here, the bond strengths

Figure 5. Density plot along two coordinates characterizing hydrogen bonding. The shaded area describes geometries corresponding to hydrogen-bonding arrangements. The densities were obtained from MD simulations at 300 K for a solute (ammonia, hydrogen peroxide, water) in a water box. The simulations of solvated molecules were performed using our in-house ABIN code combined with the CP2K package.39 The water cubic box contained 64 water molecules, and the density was set to 1 g/mL. The boxes with the solute were prepared by replacing one water molecule with the solute. The simulations were performed in the NVT ensemble. Nuclear quantum effects were incorporated via quantum thermostat-based generalized Langevin equation (GLE),40 with the parameters of the thermostat for a temperature of 298 K taken from the Web site generator (https://epflcosmo.github.io/gle4md). BLYP functional was used to calculate the energies and forces with a mixed Gaussian/plane waves basis set DZVP-MOLOPT-GTH was used,41 and a plane wave cutoff of 400 Ry was applied. The time step of the simulations was 0.5 fs, and the total length of simulations was 22 ps. The final 10 ps of the simulation were used for a subsequent analysis.

for H2O, H2O2, and NH3 in aqueous solutions are characterized by the distributions of the X−H bond versus the angle X−H− Oa as obtained from molecular dynamics simulations, where Oa is the oxygen atom of a hydrogen-bond accepting water molecule. The gray area corresponds to arrangements which are considered as hydrogen bonded according to the common definition of a hydrogen bond. This area comprises most data 239



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Figure 7. Nuclear dynamics of water, hydrogen peroxide, and ammonia in the core-ionized state for small solute−water clusters supporting hydrogen bonding. Blue lines denote regular water, red lines denote deuterated water. The densities along the X−H coordinates, X = N, O, are shown for the ground state distribution, that is, t = 0 fs and for a time corresponding to the Auger-process lifetime of either oxygen (t = 4 fs) or nitrogen (t = 7 fs). Ground-state vibrational wave function in harmonic approximation was mapped onto a phase space using Wigner transformation, and classical dynamics was then used to follow the evolution of the system. Taking the distribution of momenta into account is particularly important here, as wave packet dispersion contributes significantly to the dynamics during the first few femtoseconds. The energetics on the core-ionized states is calculated with maximum overlap method (MOM)42 at the MP2/aug-cc-pCVDZ level using Q-chem code.43 For more information see refs19 and 20.

Figure 6. (A) Oxygen-1s Auger-electron spectra from the hydrogen peroxide solute, obtained by subtracting the reference water spectrum from the 3.5 M hydrogen-peroxide(aq) solution spectrum. Results are shown for H2O2 in H2O(aq) (blue spectrum) and D2O2 in D2O(aq) (in red). The photon energy for O 1s ionization was 600 eV. The smaller intensity from local Auger decay near 504 eV in the blue spectrum reflects the competition of local and nonlocal relaxation and leads to shoveling of intensity to the high-KE tail, the region of 1h1h states (yellow-shaded). (B) Nitrogen-1s Auger-electron spectra from 2.6 M NH3 (in blue, tiers (a) and (b)) and 2.6 M ND3 (in red, tier (a); in gray, tier (b)) aqueous solution, and from 1.2 M GlyH− (in blue) and 1.2 M GlyD− (in red) aqueous solutions. Photon energy was 500 eV, that is, well above the N 1s electron binding energy of 404 eV.27 GlyH− and GlyD− refer to anionic glycine where the amino group is −NH2 and −ND2, dissolved in light and heavy water, respectively. Tier (a) displays the two spectra as measured. Local dicationic states for gas and liquid phase, 2hgas and 2haq, and nonlocal liquid-phase states, 1h1haq states (found in yellow-shaded region), are labeled. The asmeasured spectrum from ND3 (aq) (tier (a)) is seen to be shifted toward 600 meV lower kinetic energies compared to the NH3(aq) spectrum (in blue). Tier (b) shows the same spectra as in tier (a) but with a +600 meV energy shift applied to the ND3(aq) spectrum (now in red).

processes. Ammonia strongly evaporates from the solution, and the signal contribution from gas-phase ammonia is very large. From the spectrum, tier (a), we find that the isotope effect already exists for the gas phase and can be assigned to the intramolecular relaxation dynamics, namely the planarization of the NH3 pyramidal structure upon core ionization.21 When compensating for the intramolecular-relaxation component in the energy shift (Figure 5B, tier (b)), that is, aligning the respective gas-phase spectral features from the two solutions at the same energies, the isotope shift is only humble, visible between 375 and 385 eV kinetic energies (yellow-shaded in the figure). This is the spectral signature of 1h1h final states, and population is seen to be rather large. However, the isotope

process as appearing from the experimental isotope effect seems to be strong. Figure 6B, tier (a), shows the experimental N 1s Auger spectra from NH3(aqH) and ND3(aqD), of the same 2.6 M concentration, where we clearly see a relatively strong isotope effect (energy shift), but it is not only due to PTM 240



Perspective

strength of the hydrogen bonding. In principle, it should be also related to the coordination number, which can be perturbed by the presence of other solutes. There is plenty of work to be done in this direction. Among others, this includes the exploration of the PTM-CS signal for different temperatures, and possible extension into the no-man’s land,35 exploring PTM in water bound to anionic solutes, as well as investigating other hydrogen-bonded systems and their mixtures. Second, the newly discovered relaxation processes change our view on the first few femtoseconds in radiation chemistry. Local Auger process, nonlocal ICD and ETMD as well as their PTM analogues all lead to different species, either H2O2+, two H2O+ units or HO+ and H3O+ pairs, formed upon autoionization. In each case, the chemical reactivity of these initially formed molecules is different. The contribution of the nonlocal processes is clearly significant, and we argue that these processes should be considered in models of interaction of high-energy radiation with hydrated systems, allowing for corelevel ionization. Furthermore, can we track the PTM-CS and coupled autoionization spectral fingerprints to explore the protontransfer processes following core ionization of complex biomolecules? Would we be able to observe charge transfer, that is, along the hydrogen bonds in the DNA base pairs? We do not know the answer yet. Experimentally, this is a challenging task, and it would be useful to consider these processes at least from a theory point of view. Finally, if we were able to control the PTM-CS process, applications in radiotherapy can be envisioned. In this context, studies of nonlocal electron/nuclear relaxation processes, utilizing suitable nonbiogenic markers, may lead to targeted damage in the proximity of these molecules, improving medical treatment. Note however that even though the X-ray energy can be tuned to a selective excitation, the local environment is inevitably ionized because of the high photon energy. What needs to be done next? One of the major experimental challenges is to track the complex relaxation dynamics of PTMCS and subsequent autoionization in an actual time-resolved measurement with few-femtosecond resolution, currently feasible only with X-ray free-electron lasers.36 The particular benefit is that the observation window could be extended to sufficiently longer times at which follow-up chemical reactions occur. It also would be extremely useful to detect the discussed relaxation processes for the solution−solid interface. This could be the electrode−solution interface of a photo- or electrochemical cell, used for instance, for generation of solar fuel from catalyzed water hydrolysis. Perhaps, one can use the PTM-CS fingerprint of interfacial water to make predictions on the water bond-breaking. Experimentally, the detection of relatively lowkinetic energy electrons (

Relaxation Processes in Aqueous Systems upon X-ray Ionization

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