Water dynamics and sum-frequency generation spectra at electrode/aqueous electrolyte interfaces.

The dynamics of water at interfaces between an electrode and an electrolyte is essential for the transport of redox species and for the kinetics of charge transfer reactions next to the electrode. However, while the effects of electrode potential and ion concentration on the electric double layer structure have been extensively studied, a comparable understanding of dynamical aspects is missing. Interfacial water dynamics presents challenges since it is expected to result from the complex combination of water-water, water-electrode and water-ion interactions. Here we perform molecular dynamics simulations of aqueous NaCl solutions at the interface with graphene electrodes, and examine the impact of both ion concentration and electrode potential on interfacial water reorientational dynamics. We show that for all salt concentrations water dynamics exhibits strongly asymmetric behavior: it slows down at increasingly positively charged electrodes but it accelerates at increasingly negatively charged electrodes. At negative potentials water dynamics is determined mostly by the electrode potential value, but in contrast at positive potentials it is governed both by ion-water and electrode-water interactions. We show how these strikingly different behaviors are determined by the interfacial hydrogen-bond network structure and by the ions' surface affinity. Finally, we indicate how the structural rearrangements impacting water dynamics can be probed via vibrational sum-frequency generation spectroscopy.

A Model Electron Transfer Reaction in Confined Aqueous Solution.

Liquid water confined within nanometer-sized channels exhibits a strongly reduced local dielectric constant perpendicular to the wall, especially at the interface, and this has been suggested to induce faster electron transfer kinetics at the interface than in the bulk. We study a model electron transfer reaction in aqueous solution confined between graphene sheets with classical molecular dynamics. We show that the solvent reorganization energy is reduced at the interface compared to the bulk, which explains the larger rate constant. However, this facilitated solvent reorganization is due to the partial desolvation by the graphene sheet of the ions involved in the electron transfer and not to a local dielectric constant reduction effect.

Confined Water's Dielectric Constant Reduction Is Due to the Surrounding Low Dielectric Media and Not to Interfacial Molecular Ordering.

Liquid water confined within nanometer-sized channels exhibits a surprisingly low dielectric constant along the direction orthogonal to the channel walls. This is typically assumed to result from a pronounced heterogeneity across the sample: the dielectric constant would be bulk-like everywhere except at the interface, where it would be dramatically reduced by strong restrictions on interfacial molecules. Here we study the dielectric properties of water confined within graphene slit channels via classical molecular dynamics simulations. We show that the permittivity reduction is not due to any important alignment of interfacial water molecules, but instead to the long-ranged anisotropic dipole correlations combined with an excluded-volume effect of the low-dielectric confining material. The bulk permittivity is gradually recovered only over several nanometers due to the impact of long-range electrostatics, rather than structural features. This has important consequences for the control of, e.g., ion transport and chemical reactivity in nanoscopic channels and droplets.

Setting the photoelectron clock through molecular alignment.

The interaction of strong laser fields with matter intrinsically provides a powerful tool for imaging transient dynamics with an extremely high spatiotemporal resolution. Here, we study strong-field ionisation of laser-aligned molecules, and show a full real-time picture of the photoelectron dynamics in the combined action of the laser field and the molecular interaction. We demonstrate that the molecule has a dramatic impact on the overall strong-field dynamics: it sets the clock for the emission of electrons with a given rescattering kinetic energy. This result represents a benchmark for the seminal statements of molecular-frame strong-field physics and has strong impact on the interpretation of self-diffraction experiments. Furthermore, the resulting encoding of the time-energy relation in molecular-frame photoelectron momentum distributions shows the way of probing the molecular potential in real-time, and accessing a deeper understanding of electron transport during strong-field interactions.