Excited electronic states of small water clusters

Nonadiabatic Molecular Dynamics of Water Cluster Dissociation by Vacuum Ultraviolet Absorption or Electron Impact Excitation

After five decades of investigation since the 1970s, the nature of photon-induced or electron-induced water dissociation is still largely studied only in the gas phase, with a notable absence of dynamics studies of water clusters and bulk water. We study the problem with density functional theory and the nonadiabatic fewest switches surface hopping technique considering both singlet and triplet excited states to study the dissociation of water clusters leading mainly to OH + H. For clusters of 40 water molecules, the mean dissociation time was found to be <10 fs, and the threshold energy was ∼6 eV. Dissociation is almost exclusively associated with the cluster surface due to the lower energy of surface water excited states relative to the bulk. Recombination plays a major role in vacuum ultraviolet dissociation. O + H2 is found as a minor product in the dissociation and is mostly produced in "roaming" trajectories.

Occurrence of unknown reactive species in UV/H2O2 system leading to false interpretation of hydroxyl radical probe reactions

The UV/H₂O₂ process is a benchmark advanced oxidation process (AOP) that in situ generates highly reactive and nonselective hydroxyl radical (^•OH) to oxidatively destroy a wide range of organic compounds. Accurately quantifying the concentration of short-lived ^•OH is essential to predict process performance, optimize the operation parameters, and compare with other process options. The ^•OH concentration is typically measured using organic probe molecules that react with ^•OH but not with other oxidants. In the extremely well-characterized UV/H₂O₂ system in which ^•OH is proven to be the dominant oxidant, using photolysis-resistant probes such as benzoic acid and its derivatives is a widely agreed and practiced norm. We herein report that certain ^•OH probe compounds can be degraded in UV/H₂O₂ system by unknown reactive species that has not been reported in the past. Several common organic probes, particularly p-substituted benzoic acid compounds (i.e., p-hydroxybenzoic acid, p-chlorobenzoic acid, and p-phthalic acid), were found to be vulnerable to attack by the unknown reactive species, leading to false quantification of ^•OH concentration under high radical scavenging conditions. Lines of evidence obtained from a series of ^•OH scavenging experiments performed under various conditions (i.e., different concentrations of H₂O₂, ^•OH probe compounds, and dissolved oxygen) point toward excited state H₂O₂. The results from this study suggest the importance of using appropriate ^•OH probe compounds in mechanistic studies and needs for considering the unidentified role of excited state of H₂O₂ on the UV/H₂O₂ process and related AOPs.

Hydrogen Dissociation Dynamics from Water Clusters on Triplet-State Energy Surfaces

The ice surface provides two-dimensional reaction fields for bimolecular collisions in interstellar space. As H₂O molecules on the surface are typically exposed to cosmic rays, H₂O in the excited state is easily dissociated into H + OH, where a H atom is released from the surface to the gas phase. In the present study, the reaction dynamics of small-sized water clusters on the triplet-state potential energy (T₁) surface, following the vertical electronic excitation from the ground state (S₀), were investigated using direct ab initio molecular dynamics to provide insights into the generation mechanism of H atoms from an irradiated ice surface. In all clusters, that is, (H₂O)_n (n = 2-6), the H atom was directly dissociated from one of the H₂O molecules in the clusters (direct dissociation), whereas the OH radical remained in the cluster. In the branched form of H₂O tetramer (n = 4) and the book form of H₂O hexamer (n = 6), the dissociated hydrogen atom (H') collided with the neighboring H₂O molecule, and the exchange of H atoms occurred as H' + H₂O → H'-H₂O (collision) → H'OH + H (hydrogen exchange). The translational energy of the exchanged H atom decreases significantly (by approximately 50%) because the kinetic energy of the H' atom is efficiently transferred to the vibrational modes of the cluster during the H-exchange reaction. The mechanism of H atom dissociation is discussed based on theoretical results.

From atoms to aerosols: probing clusters and nanoparticles with synchrotron based mass spectrometry and X-ray spectroscopy

Synchrotron radiation provides insight into spectroscopy and dynamics in clusters and nanoparticles.

Variation of optical spectra of water clusters with size from many-body Green's function theory

Water clusters are an important species in the environment and atmosphere and take part in various chemical and biological reactions. How their optical properties vary with size is still an open question. Using the GW method and Bethe-Salpeter equation within the ab initio many-body Green's function theory, we study the electronic excitations in a series of water clusters (H₂O)_n with n = 1-48. We find that their absorption peaks blueshift with increasing cluster size due to the reducing electron-hole binding energy which arises from the enhanced electronic screening and gradually delocalized excitonic spatial distribution. The position of the first absorption peak has a close relation to the average number of hydrogen bonds per molecule. Off-diagonal matrix elements of the self-energy operator have pronounced effects on the unoccupied electronic levels and optical absorption for small clusters with n ≤ 10 when using density functional theory as the starting point for GW calculations. Although the optical absorption is predominated by delocalized excitons, highly localized excitons on a single water molecule are always present on the cluster surface in the vicinity of the absorption edge. These localized excitons may facilitate the photodissociation of water molecules. This can provide inspiration on the excited-state dynamics and photolysis in water clusters.

Electron-stimulated reactions in nanoscale water films adsorbed on α-Al2O3(0001)

100 eV electrons are stopped in the H₂O portion of the isotopically-layered nanoscale film on α-Al₂O₃(0001) but D₂ is produced at the D₂O/alumina interface by mobile electronic excitations and/or hydronium ions.

Vacuum ultraviolet spectroscopy of the lowest-lying electronic state in subcritical and supercritical water

The nature and extent of hydrogen bonding in water has been scrutinized for decades, including how it manifests in optical properties. Here we report vacuum ultraviolet absorption spectra for the lowest-lying electronic state of subcritical and supercritical water. For subcritical water, the spectrum redshifts considerably with increasing temperature, demonstrating the gradual breakdown of the hydrogen-bond network. Tuning the density at 381 °C gives insight into the extent of hydrogen bonding in supercritical water. The known gas-phase spectrum, including its vibronic structure, is duplicated in the low-density limit. With increasing density, the spectrum blueshifts and the vibronic structure is quenched as the water monomer becomes electronically perturbed. Fits to the supercritical water spectra demonstrate consistency with dimer/trimer fractions calculated from the water virial equation of state and equilibrium constants. Using the known water dimer interaction potential, we estimate the critical distance between molecules (ca. 4.5 Å) needed to explain the vibronic structure quenching.

The link between hydrogen bonding and the optical properties of water has been debated for many years, but not fully understood. Here, the authors report vacuum ultraviolet absorption spectra for subcritical and supercritical water, providing insight into the electronic structure of water and its relation to hydrogen bonding.

The mechanism for the formation of OH radicals in condensed-phase water under ultraviolet irradiation

OH radicals can be produced via direct water photolysis through concerted proton and electron transfer.

Strong enhancement of cage effects in water photolysis caused by interatomic Coulombic decay

The impact of the solvent on the photodissociation of embedded molecules has been intensively investigated in the last decades. Collisions of photofragments with the solvating atoms or molecules can change their kinetic energy distribution or even lead to the de-excitation of the dissociating molecule to a bound electronic state quenching the dissociation. In this article we show that this cage effect is strongly enhanced if interatomic Coulombic decay (ICD) of the excited state becomes allowed. Ab initio calculations in H2O-Cl(-) cluster show that the ultra-fast dissociation of water in the à excited state is strongly quenched by ICD. We found that this very efficient quenching is due to two factors. First, the lifetimes of the à state due to ICD are short ranging between 6 and 30 fs. Second, nuclear dynamics is dominated by the chattering motion of the H atom between O and Cl(-) allowing ICD to act for longer times. We hope that this work will be an important first step in clarifying the impact of ICD on photodissociation of embedded molecules.

Surface Effect of Alumina on the First Electronic Transition of Liquid Water Studied by Far-Ultraviolet Spectroscopy

The first electronic transition (à ← X̃) of liquid water (H2O and D2O) on an α-alumina substrate was studied using variable angle attenuated total reflection far-ultraviolet (VA-ATR-FUV) spectroscopy in the wavelength region 140-180 nm (8.86-6.89 eV). A variation in the penetration depth of the evanescent wave of a probe light (25-19 nm) directly determined individual FUV spectra associated with bulk water (distance from the alumina surface >2 nm) and interfacial water (<2 nm). We found that the à ← X̃ band of the interfacial water was markedly blue-shifted and red-tailed relative to the bulk water. The electronic state difference of the interfacial water from the bulk water mainly arose from the hydrogen-bond structure and energy affected by the alumina surface.

Insights into the ultraviolet spectrum of liquid water from model calculations: the different roles of donor and acceptor hydrogen bonds in water pentamers

With a view toward a better understanding of changes in the peak position and shape of the first absorption band of water with condensation or temperature, results from electronic structure calculations using high level wavefunction based and time-dependent density functional methods are reported for water pentamers. Excitation energies, oscillator strengths, and redistributions of electron density are determined for the quasitetrahedral water pentamer in its C(2v) equilibrium geometry and for many pentamer configurations sampled from molecular simulation of liquid water. Excitations associated with surface molecules are removed in order to focus on those states associated with the central molecule, which are the most representative of the liquid environment. The effect of hydrogen bonding on the lowest excited state associated with the central molecule is studied by adding acceptor or donor hydrogen bonds to tetramer and trimer substructures of the C(2v) pentamer, and by sampling liquid-like configurations having increasing number of acceptor or donor hydrogen bonds of the central molecule. Our results provide clear evidence that the blueshift of excitation energies upon condensation is essentially determined by acceptor hydrogen bonds, and the magnitudes of these shifts are determined by the number of such, whereas donor hydrogen bonds do not induce significant shifts in excitation energies. This qualitatively different role of donor and acceptor hydrogen bonds is understood in terms of the different roles of the 1b(1) monomer molecular orbitals, which establishes an intimate connection between the valence hole and excitation energy shifts. Since the valence hole of the lowest excitation associated with the central molecule is found to be well localized in all liquid-like hydrogen bonding environments, with an average radius of gyration of ~1.6 Å that is much lower than the nearest neighbor O-O distance, a clear and unambiguous connection between hydrogen bonding environments and excitation energy shifts can be established. Based on these results, it is concluded that peak position of the first absorption band is mainly determined by the relative distribution of single and double acceptor hydrogen bonding environments, whereas the shape of the first absorption band is mainly determined by the relative distribution of acceptor and broken acceptor hydrogen bonding environments. The temperature dependence of the peak position and shape of the first absorption band can be readily understood in terms of changes to these relative populations.

Excitation Energy Transfer and Low-Efficiency Photolytic Splitting of Water Ice by Vacuum UV Light

Experimental estimates of photolytic efficiency (yield per photon) for photodissociation and photodesorption from water ice range from about 10(-3) to 10(-1). However, in the case of photodissociation of water in the gas phase, it is close to unity. Exciton dynamics carried out by a quantum mechanical time-dependent propagator shows that in the eight most stable water hexamers, the excitation diffuses away from the initially excited molecule within a few femtoseconds. On the basis of these quantum dynamics simulations, it is hypothesized that the ultrafast exciton energy transfer process, which in general gives rise to a delocalized exciton within these clusters, may contribute to the low efficiency of photolytic processes in water ice. It is proposed that exciton diffusion inherently competes with the nuclear dynamics that drives the photodissociation process in the repulsive S1 state on the sub-10 fs time scale.

Thermal and nonthermal physiochemical processes in nanoscale films of amorphous solid water

Amorphous solid water (ASW) is a disordered version of ice created by vapor deposition onto a cold substrate (typically less than 130 K). It has a higher free energy than the crystalline phase of ice, and when heated above its glass transition temperature, it transforms into a metastable supercooled liquid. This unusual form of water exists on earth only in laboratories, after preparation with highly specialized equipment. It is thus fair to ask why there is any interest in studying such an esoteric material. Much of the scientific interest results from the ability to use ASW as a model system for exploring the physical and reactive properties of liquid water and aqueous solutions. ASW is also thought to be the predominant form of water in the extremely cold temperatures of many interstellar and planetary environments. In addition, ASW is a convenient model system for studying the stability of amorphous and glassy materials as well as the properties of highly porous materials. A fundamental understanding of such properties is invaluable in a diverse range of applications, including cryobiology, food science, pharmaceuticals, astrophysics, and nuclear waste storage, among others. Over the past 15 years, we have used molecular beams and surface science techniques to probe the thermal and nonthermal properties of nanoscale films of ASW. In this Account, we present a survey of our research on the properties of ASW using this approach. We use molecular beams to precisely control the deposition conditions (flux, incident energy, and incident angle) and create compositionally tailored, nanoscale films of ASW at low temperatures. To study the transport properties (viscosity and diffusivity), we heat the amorphous films above their glass transition temperature, T(g), at which they transform into deeply supercooled liquids prior to crystallization. The advantage of this approach is that at temperatures near T(g), the viscosity is approximately 15 orders of magnitude larger than that of a normal liquid. As a result, the crystallization kinetics are dramatically slowed, increasing the time available for experiments. For example, near T(g), a water molecule moves less than the distance of a single molecule on a typical laboratory time scale (∼1000 s). For this reason, nanoscale films help to probe the behavior and reactions of supercooled liquids at these low temperatures. ASW films can also be used for investigating the nonthermal reactions relevant to radiolysis.

Insights into the ultraviolet spectrum of liquid water from model calculations

With a view toward a better molecular level understanding of the effects of hydrogen bonding on the ultraviolet absorption spectrum of liquid water, benchmark electronic structure calculations using high level wave function based methods and systematically enlarged basis sets are reported for excitation energies and oscillator strengths of valence excited states in the equilibrium water monomer and dimer and in a selection of liquid-like dimer structures. Analysis of the electron density redistribution associated with the two lowest valence excitations of the water dimer shows that these are usually localized on one or the other monomer, although valence hole delocalization can occur for certain relative orientations of the water molecules. The lowest excited state is mostly associated with the hydrogen bond donor and the significantly higher energy second excited state mostly with the acceptor. The magnitude of the lowest excitation energies is strongly dependent on where the valence hole is created, and only to a lesser degree on the perturbation of the excited electron density distribution by the neighboring water molecule. These results suggest that the lowest excitation energies in clusters and liquid water can be associated with broken acceptor hydrogen bonds, which provide energetically favorable locations for the formation of a valence hole. Higher valence excited states of the dimer typically involve delocalization of the valence hole and/or delocalization of the excited electron and/or charge transfer. Two of the higher valence excited states that involve delocalized valence holes always have particularly large oscillator strengths. Due to the pervasive delocalization and charge transfer, it is suggested that most condensed phase water valence excitations intimately involve more than one water molecule and, as a consequence, will not be adequately described by models based on perturbation of free water monomer states. The benchmark calculations are further used to evaluate a series of representative semilocal, global hybrid, and range separated hybrid functionals used in efficient time-dependent density functional methods. It is shown that such an evaluation is only meaningful when comparison is made at or near the complete basis set limit of the wave function based reference method. A functional is found that quantitatively describes the two lowest excitations of water dimer and also provides a semiquantitative description of the higher energy valence excited states. This functional is recommended for use in further studies on the absorption spectrum of large water clusters and of condensed phase water.

Desorption of hydroxyl radicals in the vacuum ultraviolet photolysis of amorphous solid water at 90 K

We have studied the desorption dynamics of OH radicals from the 157 nm photodissociation of amorphous solid water (ASW) as well as H(2)O(2) deposited on an ASW surface at 90 K. The translational and internal energy distributions of OH were measured using resonance-enhanced multiphoton ionization methods. These distributions are compared to reported molecular dynamics calculations for the condensed phase photodissociation of water ice and also reported results for the gas phase photodissociation of H(2)O at 157 nm. We have confirmed that OH radicals are produced from two different mechanisms: one from primary photolysis of surface H(2)O of ASW, and the other being secondary photolysis of H(2)O(2) photoproducts on the ASW surface after prolonged irradiation at 157 nm.

Degree of initial hole localization/delocalization in ionized water clusters

The electronic structure of ionized bulk liquid water presents a number of theoretical challenges. Not the least of these is the realization that the detailed geometry of the hydrogen bonding network is expected to have a strong effect on the electronic couplings between water molecules and thus the degree of delocalization of the initially ionized system. This problem is approached from a cluster perspective where a high-level coupled cluster description of the electronic structure is still possible. Building on the work and methodology developed for the water dimer cation [J. Phys. Chem. A 2008, 112, 6159], the character and spectrum of electronic states of the water hole and their evolution from the dimer into higher clusters is presented. As the time evolution of the initially formed hole can in principle be followed by the system's transient absorption spectrum, the state spacings and transition strengths are computed. An analysis involving Dyson orbitals is applied and shows a partially delocalized nature of states. The issue of conformation disorder in the hydrogen bonding geometry is addressed for the water dimer cation.

Excited state properties of liquid water

In this paper, we give an overview of the state of the art in calculations of the electronic band structure and absorption spectra of water. After an introduction to the main theoretical and computational schemes used, we present results for the electronic and optical excitations of water. We focus mainly on liquid water, but spectroscopic properties of ice and vapor phase are also described. The applicability and the accuracy of first-principles methods are discussed, and results are critically presented.

Resolving the optical spectrum of water: coordination and electrostatic effects

The optical absorption of small water clusters, water chains, liquid water, and crystalline ice is analyzed computationally. We identify two competing mechanisms determining the onset of the optical absorption: Electronic transitions involving surface molecules of finite clusters or chains cause a redshift upon molecular aggregation compared to monomers. On the other hand, a strong blueshift is caused by the electrostatic environment experienced by water monomers embedded in a hydrate shell. Concerning the recent dispute over the structure of the liquid, the present results support the conventional fourfold coordinated water, as obtained from ab initio molecular-dynamics simulations.

Temperature and density dependence of the light and heavy water ultraviolet absorption edge

Characteristics of the ultraviolet absorption band edge of high-pressure light and heavy water are reported over the temperature range of 25-400 degrees C, extending into the supercritical regime. A gradual redshift in the absorption band edge of approximately 0.6 eV is observed with increasing temperature. This shift cannot be explained by vibrational hot band growth or changes in the degree of Rayleigh scattering with increasing temperature, and is ascribed to a shift of the electronic transition energy. The density dependence for the absorption edge in 400 degrees C supercritical water was also examined, and showed a surprising approximately 0.1 eV blueshift over a factor of 3.5 decrease in density. This shift may be due to a narrowing of the absorption spectrum with decreasing density. It is proposed that the previously reported "red tail" of the water absorption extending into the near ultraviolet and visible could be attributed to preresonant Rayleigh scattering, and that the true onset of liquid water absorption is approximately 5.8 eV at 25 degrees C.

Excited state dynamics of liquid water: Insight from the dissociation reaction following two-photon excitation

The authors use transient absorption spectroscopy to monitor the ionization and dissociation products following two-photon excitation of pure liquid water. The primary decay mechanism changes from dissociation at an excitation energy of 8.3 eV to ionization at 12.4 eV. The two channels occur with similar yield for an excitation energy of 9.3 eV. For the lowest excitation energy, the transient absorption at 267 nm probes the geminate recombination kinetics of the H and OH fragments, providing a window on the dissociation dynamics. Modeling the OH geminate recombination indicates that the dissociating H atoms have enough kinetic energy to escape the solvent cage and one or two additional solvent shells. The average initial separation of H and OH fragments is 0.7+/-0.2 nm. Our observation suggests that the hydrogen bonding environment does not prevent direct dissociation of an O-H bond in the excited state. We discuss the implications of our measurement for the excited state dynamics of liquid water and explore the role of those dynamics in the ionization mechanism at low excitation energies.

Electronically excited water aggregates and the adiabatic band gap of water

The authors report results for the electronic properties of the S(1) singlet excited state of H(3)O(+)[OH(H(2)O)((n-2))](-) aggregates, where n=2-5 is the number of water molecules. The energy of the excited state was defined relative to the ground state of neutral water clusters. Results for ionized aggregates are also reported. The results are based on configuration interaction with single excitations geometry optimizations followed by density functional theory and time dependent density functional theory calculations. Emphasis was placed on the relationship between electronic properties of the aggregates and the adiabatic band gap of liquid water. The authors' predictions for the adiabatic energy gap of water clusters are in the 6.01-6.55 eV range. These values are approximately 0.9-0.4 eV below the experimental adiabatic band gap of liquid water (6.9 eV). Comparison with experimental information for water is reported for several properties of interest including vertical and adiabatic ionization energies, excitation energies, photoemission thresholds, and conduction band edge. The results provide a description, at the molecular level, for the electronic properties of water aggregates that is consistent with the current picture for the band gap of bulk water [J. V. Coe, Int. Rev. Phys. Chem. 20, 33 (2001)].

Computational studies of aqueous-phase photochemistry and the hydrated electron in finite-size clusters

A survey of recent ab initio calculations on excited electronic states of water clusters and various chromophore-water clusters is given. Electron and proton transfer processes in these systems have been characterized by the determination of electronic wave functions, minimum-energy reaction paths and potential-energy profiles. It is pointed out that the transfer of a neutral hydrogen atom (leading to biradicals) rather than the transfer of a proton (leading to ion pairs) is the generic excited-state reaction mechanism in these systems. The hydrated hydronium radical, (H3O)(aq), plays a central role in this scenario. The electronic and vibrational spectra of H3O(H2O)(n) clusters and the decay mechanism of these metastable species have been investigated in some detail. The results suggest that (H3O)(aq) could be the carrier of the characteristic spectroscopic properties of the hydrated electron in liquid water.