Terahertz Spectroscopy of Aqueous Solutions of Alkali (M) Chlorides Using Polarizable Models: Self- and Cross-Correlation Contributions of Ions and Hydration Shell Water for M⁺ = Li⁺, Na⁺, K⁺, Rb<sup>+</sup

We have investigated the terahertz (THz) absorption spectra of aqueous alkali (M) chloride solutions for all five alkali metal ions (M⁺ = Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) using polarizable models for both water and ions. The difference THz spectrum is calculated for each of the solutions, and various features appearing below 100 cm^-1 and at around 200 cm^-1 are connected to the underlying motion of ions and water in the hydration shells. The spectrum for a given solution is subsequently dissected into self- and cross-contributions and also into ion-ion, water-water, and ion-water cross-correlation contributions. Further decomposition of these components into contributions from permanent charges/dipoles and induced dipoles is carried out, which provides useful information regarding the origin of various features of the difference THz spectra at different frequencies. The current calculations are further extended to study the length-scale-resolved THz responses of ion-water correlations from different solvation shells, which shed light on how far the ion effects continue and to what extent such length scale dependence varies with the size and charge density of the ions.