Photochemistry of water: The (H2O)5 cluster

Pickup and reactions of molecules on clusters relevant for atmospheric and interstellar processes

In this perspective, we review experiments with molecules picked up on large clusters in molecular beams with the focus on the processes in atmospheric and interstellar chemistry. First, we concentrate on the pickup itself, and we discuss the pickup cross sections. We measure the uptake of different atmospheric molecules on mixed nitric acid-water clusters and determine the accommodation coefficients relevant for aerosol formation in the Earth's atmosphere. Then the coagulation of the adsorbed molecules on the clusters is investigated. In the second part of this perspective, we review examples of different processes triggered by UV-photons or electrons in the clusters with embedded molecules. We start with the photodissociation of hydrogen halides and Freon CF₂Cl₂ on ice nanoparticles in connection with the polar stratospheric ozone depletion. Next, we mention reactions following the excitation and ionization of the molecules adsorbed on clusters. The first ionization-triggered reaction observed between two different molecules picked up on the cluster was the proton transfer between methanol and formic acid deposited on large argon clusters. Finally, negative ion reactions after slow electron attachment are illustrated by two examples: mixed nitric acid-water clusters, and hydrogen peroxide deposited on large Ar_N and (H₂O)_N clusters. The selected examples are discussed from the perspective of the atmospheric and interstellar chemistry, and several future directions are proposed.

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.

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.

Photodissociation of hydrogen halide molecules on free ice nanoparticles

Photodissociation of water clusters doped with HX(X=Br,Cl), molecules has been studied in a molecular beam experiment. The HX(H2O)n clusters are dissociated with 193 nm laser pulses, and the H fragments are ionized at 243.07 nm and their time-of-flight distributions are measured. Experiments with deuterated species DBr(H2O)n and HBr(D2O)n suggest that the photodissociation signal originates from the presence of the HX molecule on the water cluster, but does not come directly from a photolysis of the HX molecule. The H fragment is proposed to originate from the hydronium molecule H3O. Possible mechanisms of the H3O production are discussed. Experimental evidence suggests that acidic dissociation takes place in the cluster, but the H3O+ ion remains rather immobile.

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.

Photodissociation of polycrystalline and amorphous water ice films at 157 and 193nm

The photodissociation dynamics of amorphous solid water (ASW) films and polycrystalline ice (PCI) films at a substrate temperature of 100 K have been investigated by analyzing the time-of-flight (TOF) mass spectra of photofragment hydrogen atoms at 157 and 193 nm. For PCI films, the TOF spectrum recorded at 157 nm could be characterized by a combination of three different (fast, medium, and slow) Maxwell-Boltzmann energy distributions, while that measured at 193 nm can be fitted in terms of solely a fast component. For ASW films, the TOF spectra measured at 157 and 193 nm were both dominated by the slow component, indicating that the photofragment H atoms are accommodated to the substrate temperature by collisions. H atom formation at 193 nm is attributed to the photodissociation of water species on the ice surface, while at 157 nm it is ascribable to a mixture of surface and bulk photodissociations. Atmospheric implications in the high latitude mesopause region of the Earth are discussed.

Molecular-dynamics study of photodissociation of water in crystalline and amorphous ices

We present the results of classical dynamics calculations performed to study the photodissociation of water in crystalline and amorphous ice surfaces at a surface temperature of 10 K. A modified form of a recently developed potential model for the photodissociation of a water molecule in ice [S. Andersson et al., Chem. Phys. Lett. 408, 415 (2005)] is used. Dissociation in the top six monolayers is considered. Desorption of H(2)O has a low probability (less than 0.5% yield per absorbed photon) for both types of ice. The final outcome strongly depends on the original position of the photodissociated molecule. For molecules in the first bilayer of crystalline ice and the corresponding layers in amorphous ice, desorption of H atoms dominates. In the second bilayer H atom desorption, trapping of the H and OH fragments in the ice, and recombination of H and OH are of roughly equal importance. Deeper into the ice H atom desorption becomes less important and trapping and recombination dominate. Motion of the photofragments is somewhat more restricted in amorphous ice. The distribution of distances traveled by H atoms in the ice peaks at 6-7 Angstroms with a tail going to about 60 Angstroms for both types of ice. The mobility of OH radicals is low within the ice with most probable distances traveled of 2 and 1 Angstrom for crystalline and amorphous ices, respectively. OH is, however, quite mobile on top of the surface, where it has been found to travel more than 80 Angstroms. Simulated absorption spectra of crystalline ice, amorphous ice, and liquid water are found to be in very good agreement with the experiments. The outcomes of photodissociation in crystalline and amorphous ices are overall similar, but with some intriguing differences in detail. The probability of H atoms desorbing is 40% higher from amorphous than from crystalline ice and the kinetic-energy distribution of the H atoms is on average 30% hotter for amorphous ice. In contrast, the probability of desorption of OH radicals from crystalline ice is much higher than that from amorphous ice.

The World of Non-Covalent Interactions: 2006

The review focusses on the fundamental importance of non-covalent interactions in nature by illustrating specific examples from chemistry, physics and the biosciences. Laser spectroscopic methods and both ab initio and molecular modelling procedures used for the study of non-covalent interactions in molecular clusters are briefly outlined. The role of structure and geometry, stabilization energy, potential and free energy surfaces for molecular clusters is extensively discussed in the light of the most advanced ab initio computational results for the CCSD(T) method, extrapolated to the CBS limit. The most important types of non-covalent complexes are classified and several small and medium size non-covalent systems, including H-bonded and improper H-bonded complexes, nucleic acid base pairs, and peptides and proteins are discussed with some detail. Finally, we evaluate the interpretation of experimental results in comparison with state of the art theoretical models: this is illustrated for phenol...Ar, the benzene dimer and nucleic acid base pairs. A review with 270 references.