Electrons In Water Clusters Research Articles

Can Current Experimental Data Exclude Non-Gaussian Genuine Band Shapes in Ultraviolet Photoelectron Spectra of the Hydrated Electron?

Two recent articles present results that allegedly exclude a possible multimodal distribution of the hydrated electron in ultraviolet photoelectron spectra. The first article bases its conclusion on the assumption that the non-Gaussian genuine band shape previously retrieved for the solvated electron in liquid water is an artifact arising from insufficient electron scattering cross sections used in the retrieval. The second article excludes a multimodal band shape based on a photoelectron spectrum of the solvated electron in water clusters recorded at a single ultraviolet photon energy, and it further assumes that cluster results are transferable to the liquid without further justification. Here, we show that based on current data multimodal distributions cannot be unambiguously excluded. Furthermore, the transferability of cluster results to the liquid can be neither justified nor refuted on the basis of currently available experimental ultraviolet photoelectron spectra.

Hydrated Electrons in Water Clusters: Inside or Outside, Cavity or Noncavity?

In this work, we compare the applicability of three electron–water molecule pseudopotentials in modeling the physical properties of hydrated electrons. Quantum model calculations illustrate that the recently suggested Larsen–Glover–Schwartz (LGS) model and its modified m-LGS version have a too-attractive potential in the vicinity of the oxygen. As a result, LGS models predict a noncavity hydrated electron structure in clusters at room temperature, as seen from mixed one-electron quantum–classical molecular dynamics simulations of water cluster anions, with the electron localizing exclusively in the interior of the clusters. Comparative calculations using the cavity-preferring Turi–Borgis (TB) model predict interior-state and surface-state cluster isomers. The computed associated physical properties are also analyzed and compared to available experimental data. We find that the LGS and m-LGS potentials provide results that appear to be inconsistent with the size dependence of the experimental data. The simulated TB tendencies are qualitatively correct. Furthermore, ab initio calculations on static LGS noncavity structures indicate weak stabilization of the excess electron in regions where the LGS potential preferably and strongly binds the electron. TB calculations give stabilization energies that are in line with the ab initio results. In conclusion, we observe that the cavity-preferring pseudopotential model predicts cluster physical properties in better agreement with experimental data and ab initio calculations than the models predicting noncavity structures for the hydrated electron.

A New Look at Electrons in Water Clusters Solves a Longstanding Riddle

Influenced by cluster size and temperature, an electron can attach to the surface or be trapped internally.

Theoretical studies of the spectroscopy of excess electrons in water clusters

Variational calculation based on a continuum dielectric model, and numerical simulations based on the RWK2-M water potential and on a pseudopotential for the electron–water interaction, are used to evaluate excitation energies and optical spectra for bound interior states of an excess electron in water clusters and in bulk water. Additionally, optical data for surface states are obtained from numerical simulations. The simulation approach uses adiabatic dynamics based on the quantum-classical time-dependent self-consistent field (TDSCF) approximation and the fast-Fourier transform (FFT) algorithm for solving the Schrödinger equation. Both approaches predict very weak or no cluster size dependence of the excitation spectrum for clusters that support interior solvated electron states. For an electron attached to the cluster in a surface localization mode, bound excited states exist for most nuclear configurations of clusters down to (H2O)−18, and the corresponding excitation energy is strongly shifted to the red relative to that associated with stable internal states in larger clusters. Binding and excitation energies associated with surface states are about half the value of these quantities for interior states. The present variational continuum dielectric theory is in relatively good agreement with the simulation results on the size dependence of the relative stability of interior states. However, it strongly underestimates the vertical excitation energy of the solvated electron. It is suggested that optical spectroscopy of excess electrons in water clusters could serve as a sensitive probe of the transition from surface to interior localization modes as the number of water molecules in the cluster is increased.

Dynamics and spectra of a solvated electron in water clusters

The dynamics and spectra of negatively charged water clusters, containing a single excess electron, are investigated. In our calculations the atomic water constituents of the clusters are treated classically while the excess electron is described quantum mechanically using the fast Fourier transform algorithm to solve the Schrödinger equation. Information about ground and excited electronic states corresponding to the equilibrium, finite temperature, ground-state ensemble configurations can be obtained by solving for these states for given nuclear configurations generated via quantum mechanical path-integral molecular dynamics simulations. As an alternative, more efficient way, we introduce the adiabatic simulation method which consists of propagating the nuclei in real time while concurrently annealing the electronic wave functions to their correct values corresponding to the instantaneous, dynamically generated nuclear configurations. The resulting trajectories can be used for analyzing nuclear motion in the ground electronic state as well as for calculating energy distributions for the ground and excited electronic states and the (vertical) excitation line shape. We study the cluster size effect on these quantities, and in particular, by comparing results for(H2O)−64 and (H2O)−128, we conclude that the vertical ionization potential increases while the vertical excitation energy to the bound excited state decreases for larger cluster sizes. For the smallest negatively charged water cluster (H2O)−2, where adiabatic separation of electronic and nuclear motion does not hold, we simulate the time evolution in the TDSCF approximation. The dynamics reveals the close correlation between the electronic binding energy and the cluster dipole, and provides information on intramolecular and intermolecular vibrational motion. Comparison of vibrational density of states evaluated from the nuclear trajectories of the negatively charged and the neutral dimer shows that most of the modes associated with intermolecular motions shift to the red upon electron attachment (a few modes, possibly those associated directly with the magnitude of the total molecular dipole, shift to the blue).

ChemInform Abstract: Localization of an Excess Electron in Water Clusters

Simulation of e−–(H20)n for n=1,2,3 done at 5 K, using path integral Monte Carlo methods shows that a water dimer as well as a water trimer in the linear configuration can bind an electron in a diffuse surface state. The binding energy of the electron dimer and the electron trimer is estimated to be between 3–6 and 4–9 meV, respectively. The results indicate that the electron does not alter the structure of the water dimer but does induce observable changes in the water trimer. It is also shown that an electron does not bind to a monomer. These results are discussed in connection with recent molecular beam experiments.

Localization of an excess electron in water clusters

Simulation of e−–(H20)n for n=1,2,3 done at 5 K, using path integral Monte Carlo methods shows that a water dimer as well as a water trimer in the linear configuration can bind an electron in a diffuse surface state. The binding energy of the electron dimer and the electron trimer is estimated to be between 3–6 and 4–9 meV, respectively. The results indicate that the electron does not alter the structure of the water dimer but does induce observable changes in the water trimer. It is also shown that an electron does not bind to a monomer. These results are discussed in connection with recent molecular beam experiments.

Field detachment of the negatively charged water dimer

Beams of Ar m (H2O) 2 − ,m=0 to 4, are passed through an electric field and separated with a quadrupole mass spectrometer. Form<=2 the signal disappears at 31±1 kV/cm. Form=3 and 4 and larger (H2O) n − (n≧6) no field detachment is observed up to 40 kV/cm. The application of these results to the binding of excess electron in water clusters is discussed.