Probing the Structural Evolution of the Hydrated Electron in Water Cluster Anions (H2O)n-, n ≤ 200, by Electronic Absorption Spectroscopy

Spectroscopy and dynamics of the hydrated electron at the water/air interface

The hydrated electron, e-(aq), has attracted much attention as a central species in radiation chemistry. However, much less is known about e-(aq) at the water/air surface, despite its fundamental role in electron transfer processes at interfaces. Using time-resolved electronic sum-frequency generation spectroscopy, the electronic spectrum of e-(aq) at the water/air interface and its dynamics are measured here, following photo-oxidation of the phenoxide anion. The spectral maximum agrees with that for bulk e-(aq) and shows that the orbital density resides predominantly within the aqueous phase, in agreement with supporting calculations. In contrast, the chemistry of the interfacial hydrated electron differs from that in bulk water, with e-(aq) diffusing into the bulk and leaving the phenoxyl radical at the surface. Our work resolves long-standing questions about e-(aq) at the water/air interface and highlights its potential role in chemistry at the ubiquitous aqueous interface.

Creation and Reaction of Solvated Electrons at and near the Surface of Water

Solvated electrons (e_s^-) are among nature's most powerful reactants, with over 2600 reactions investigated in bulk water. These electrons can also be created at and near the surface of water by exposing an aqueous microjet in vacuum to gas-phase sodium atoms, which ionize into e_s^- and Na⁺ within the top few layers. When a reactive surfactant is added to the jet, the surfactant and e_s^- become coreactants localized in the interfacial region. We report the reaction of e_s^- with the surfactant benzyltrimethylammonium in a 6.7 M LiBr/water microjet at 235 K and pH = 2. The reaction intermediates trimethylamine (TMA) and benzyl radical are identified by mass spectrometry after they evaporate from solution into the gas phase. Their detection demonstrates that TMA can escape before it is protonated and benzyl before it combines with itself or a H atom. Diffusion-reaction calculations indicate that e_s^- reacts on average within 20 Å of the surface and perhaps within the surfactant monolayer itself, while unprotonated TMA evaporates from the top 40 Å. The escape depth exceeds 1300 Å for the more slowly reacting benzyl radical. These proof-of-principle experiments establish an approach for exploring the near-interfacial analogues of aqueous bulk-phase radical chemistry through the evaporation of reaction intermediates into the gas phase.

Thermally Activated vs. Photochemical Hydrogen Evolution Reactions-A Tale of Three Metals

Molecular processes behind hydrogen evolution reactions can be quite complex. In macroscopic electrochemical cells, it is extremely difficult to elucidate and understand their mechanism. Gas phase models, consisting of a metal ion and a small number of water molecules, provide unique opportunities to understand the reaction pathways in great detail. Hydrogen evolution in clusters consisting of a singly charged metal ion and one to on the order of 50 water molecules has been studied extensively for magnesium, aluminum and vanadium. Such clusters with around 10-20 water molecules are known to eliminate atomic or molecular hydrogen upon mild activation by room temperature black-body radiation. Irradiation with ultraviolet light, by contrast, enables hydrogen evolution already with a single water molecule. Here, we analyze and compare the reaction mechanisms for hydrogen evolution on the ground state as well as excited state potential energy surfaces. Five distinct mechanisms for evolution of atomic or molecular hydrogen are identified and characterized.

Surface or Internal Hydration - Does It Really Matter?

The precise location of an ion or electron, whether it is internally solvated or residing on the surface of a water cluster, remains an intriguing question. Subtle differences in the hydrogen bonding network may lead to a preference for one or the other. Here we discuss spectroscopic probes of the structure of gas-phase hydrated ions in combination with quantum chemistry, as well as H/D exchange as a means of structure elucidation. With the help of nanocalorimetry, we look for thermochemical signatures of surface vs internal solvation. Examples of strongly size-dependent reactivity are reviewed which illustrate the influence of surface vs internal solvation on unimolecular rearrangements of the cluster, as well as on the rate and product distribution of ion-molecule reactions.

Nonadiabatic Dynamics Studied by Liquid-Jet Time-Resolved Photoelectron Spectroscopy

The development of the liquid microjet technique by Faubel and co-workers has enabled the investigation of high vapor pressure liquids and solutions utilizing high-vacuum methods. One such method is photoelectron spectroscopy (PES), which allows one to probe the electronic properties of a sample through ionization in a state-specific manner. Liquid microjets consisting of pure solvents and solute-solvent systems have been studied with great success utilizing PES and, more recently, time-resolved PES (TRPES). Here, we discuss progress made over recent years in understanding the solvation and excited state dynamics of the solvated electron and nucleic acid constituents (NACs) using these methods, as well as the prospect for their future.The solvated electron is of particular interest in liquid microjet experiments as it represents the simplest solute system. Despite this simplicity, there were still many unresolved questions about its binding energy and excited state relaxation dynamics that are ideal problems for liquid microjet PES. In the work discussed in this Account, accurate binding energies were measured for the solvated electron in multiple high vapor pressure solvents. The advantages of liquid jet PES were further highlighted in the femtosecond excited state relaxation studies on the solvated electron in water where a 75 ± 20 fs lifetime attributable to internal conversion from the excited p-state to a hot ground state was measured, supporting a nonadiabatic relaxation mechanism.Nucleic acid constituents represent a class of important solutes with several unresolved questions that the liquid microjet PES method is uniquely suited to address. As TRPES is capable of tracking dynamics with state-specificity, it is ideal for instances where there are multiple excited states potentially involved in the dynamics. Time-resolved studies of NAC relaxation after excitation using ultraviolet light identified relaxation lifetimes from multiple excited states. The state-specific nature of the TRPES method allowed us to identify the lack of any signal attributable to the ¹nπ* state in thymine derived NACs. The femtosecond time resolution of the technique also aided in identifying differences between the excited state lifetimes of thymidine and thymidine monophosphate. These have been interpreted, aided by molecular dynamics simulations, as an influence of conformational differences leading to a longer excited state lifetime in thymidine monophosphate.Finally, we discuss advances in tabletop light sources extending into the extreme ultraviolet and soft X-ray regimes that allow expansion of liquid jet TRPES to full valence band and potentially core level studies of solutes and pure liquids in liquid microjets. As most solutes have ground state binding energies in the range of 10 eV, observation of both excited state decay and ground state recovery using ultraviolet pump-ultraviolet probe TRPES has been intractable. With high-harmonic generation light sources, it will be possible to not only observe complete relaxation pathways for valence level dynamics but to also track dynamics with element specificity by probing core levels of the solute of interest.

Quantum simulations of neutral water clusters and singly-charged water cluster anions

We report a computational study of the structural and energetic properties of water clusters and singly-charged water cluster anions containing from 20 to 573 water molecules. We have used both a classical and a quantum description of the molecular degrees of freedom. Water intra and inter-molecular interactions have been modelled through the SPC/F model, while the water-excess electron interaction has been described via the well-known Turi-Borgis potential. We find that in general the quantum effects of the water degrees of freedom are small, but they do influence the cluster-size at which the excess electron stabilises inside the cluster, which occurs at smaller cluster sizes when quantum effects are taken into consideration.

Photochemical Hydrogen Evolution at Metal Centers Probed with Hydrated Aluminium Cations, Al+ (H2 O)n , n=1-10

Hydrated aluminium cations have been investigated as a photochemical model system with up to ten water molecules by UV action spectroscopy in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Intense photodissociation was observed starting at 4.5 eV for two to eight water molecules with loss of atomic hydrogen, molecular hydrogen and water molecules. Quantum chemical calculations for n=2 reveal that solvation shifts the intense 3s-3p excitations of Al+ into the investigated photon energy range below 5.5 eV. During the photochemical relaxation, internal conversion from S1 to T2 takes place, and photochemical hydrogen formation starts on the T2 surface, which passes through a conical intersection, changing to T1 . On this triplet surface, the electron that was excited to the Al 3p orbital is transferred to a coordinated water molecule, which dissociates into a hydroxide ion and a hydrogen atom. If the system remains in the triplet state, this hydrogen radical is lost directly. If the system returns to singlet multiplicity, the reaction may be reversed, with recombination with the hydroxide moiety and electron transfer back to aluminium, resulting in water evaporation. Alternatively, the hydrogen radical can attack the intact water molecule, forming molecular hydrogen and aluminium dihydroxide. Photodissociation is observed for up to n=8. Clusters with n=9 or 10 occur exclusively as HAlOH+ (H2 O)n-1 and are transparent in the investigated energy range. For n=4-8, a mixture of Al+ (H2 O)n and HAlOH+ (H2 O)n-1 is present in the experiment.

Photochemistry and UV/vis spectroscopy of hydrated vanadium cations, V+(H2O)n, n = 1-41, a model system for photochemical hydrogen evolution

Photochemical hydrogen evolution provides fascinating perspectives for light harvesting. Hydrated metal ions in the gas phase are ideal model systems to study elementary steps of this reaction on a molecular level. Here we investigate mass-selected hydrated monovalent vanadium ions, with a hydration shell ranging from 1 to 41 water molecules, by photodissociation spectroscopy. The most intense absorption bands correspond to 3d-4p transitions, which shift to the red from n = 1 to n = 4, corresponding to the evolution of a square-planar complex. Additional water molecules no longer interact directly with the metal center, and no strong systematic shift is observed in larger clusters. Evolution of atomic and molecular hydrogen competes with loss of water molecules for all V+(H2O)n, n ≤ 12. For n ≥ 15, no absorptions are observed, which indicates that the cluster ensemble is fully converted to HVOH+(H2O)n-1. For the smallest clusters, the electronic transitions are modeled using multireference methods with spin-orbit coupling. A large number of quintet and triplet states is accessible, which explains the broad features observed in the experiment. Water loss most likely occurs after a series of intersystem crossings and internal conversions to the electronic ground state or a low-lying quintet state, while hydrogen evolution is favored in low lying triplet states.

Size-Resolved Electron Solvation in Neutral Water Clusters

Cluster-size-resolved ultrafast dynamics of the solvated electron in neutral water clusters with n = 3 to ∼200 molecules are studied with pump–probe time-of-flight mass spectrometry after below band gap excitation. For the smallest clusters, no longer-lived (>100–200 fs) hydrated electrons were detected, indicating a minimum size of n ∼ 14 for being able to sustain hydrated electrons. Larger clusters show a systematic increase of the number of hydrated electrons per molecule on the femtosecond to picosecond time scale. We propose that with increasing cluster size the underlying dynamics is governed by more effective electron formation processes combined with less effective electron loss processes, such as ultrafast hydrogen ejection and recombination. It appears unlikely that any size dependence of the solvent relaxation dynamics would be reflected in the observed time-resolved ion yields.

Spectroscopy and photochemistry of copper nitrate clusters

The investigation of copper nitrate cluster anions Cu(ii)n(NO3)2n+1-, n ≤ 4, in the gas phase using ultraviolet/visible/near-infrared (UV/vis/NIR) spectroscopy provides detailed insight into the electronic structure of the copper salt and its intriguing photochemistry. In the experimentally studied region up to 5.5 eV, the spectra of copper(ii) nitrate exhibit a 3d-3d band in the vis/NIR and well-separated bands in the UV. The latter bands originate from Ligand-to-Metal Charge Transfer (LMCT) as well as n-π* transitions in the nitrate ligands. The clusters predominantly decompose by loss of neutral copper nitrate in the electronic ground state after internal conversion or via the photochemical loss of a neutral NO3 ligand after a LMCT. These two decomposition channels are in direct competition on the ground state potential energy surface for the smallest copper nitrate cluster, Cu(ii)(NO3)3-. Here, copper nitrate evaporation is thermochemically less favorable. Population of π* orbitals in the nitrate ligands may lead to N-O bond photolysis. This is observed in the UV region with a small quantum efficiency, with photochemical loss of either nitrogen dioxide or an oxygen atom.

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.

IR multiple photon dissociation spectroscopy of MO2+ (M = V, Nb, Ta)

A laser vaporization cluster source is coupled to the Fourier-transform ion cyclotron resonance mass spectrometer beamline of the free-electron laser for intracavity experiments. Gas phase metal ions and their oxides (VO₂ ⁺, NbO₂ ⁺, and TaO₂ ⁺) are formed and spectroscopically characterized using IR multiple-photon dissociation spectroscopy via loss of atomic oxygen and overcoming fragmentation energies of 3 eV-6 eV. The signal is observed for all MO₂ ⁺ fundamental modes: the symmetric and anti-symmetric ν₁ and ν₃ stretch modes in the 900 cm^-1-1000 cm^-1 range and the ν₂ bending mode in the 300 cm^-1-450 cm^-1 range. A remarkable substructure is observed for the bending vibration, which is at least partly due to the rovibrational substructure.

Intrinsic electronic spectra of cryogenically prepared protoporphyrin IX ions in vacuo – deprotonation-induced Stark shifts

We present electronic spectra containing the Q_x and Q_y absorption bands of singly and doubly deprotonated protoporphyrin IX, prepared as mass selected ions in vacuo at cryogenic temperatures, revealing vibronic structure of both bands.