Terahertz Driven Ultrafast Energy Dissipation in Aqueous Ionic Solutions

Abstract Solvation of ions changes the physical, chemical and thermodynamic properties of water. The microscopic origin of this process is believed to be the ion-induced perturbation in the structure and dynamics of the hydrogen (H)-bonding network of water. Here, we provide microscopic insight on the local structural deformation of the H-bonding network of water by ions, via investigating the dissipation of external energy in salt solutions by a novel time-resolved terahertz (THz)-Raman spectroscopy. We resonantly drive the low-frequency rotational dynamics of water molecules by intense THz pulses and probe the Raman response of their intermolecular translational motions. We find that the intermolecular rotational-to-translational energy transfer is enhanced by highly-charged cations and it is drastically reduced by highly-charged anions, scaling with the ion surface charge density and concentration. Our molecular dynamics simulations further reveal that the water-water H-bond strength between the first and the second solvation shells of cations (anions) increases (decreases), signifying the opposite effects of cations and anions on the local structure of water. The impact of ion polarity on the ultrafast energy dissipation in water, resembles the effect of ions on stabilization and denaturation of proteins.