Kinetics and symmetry studies of the inner hydration sphere of Eu 3+ ion using environment-sensitive laser excitation spectroscopy

Abstract

The sharp absorption lines of Eu 3+, studied with environment-sensitive laser excitation spectroscopy, are used to describe the properties of the inner hydration sphere of this ion. The spectral bandwidth is Lorentzian for the aqueous ion and correlates with the motions of ligands within the inner hydration sphere. It is narrowed by anion interactions with Eu 3+ in the inner hydration sphere (NO − 3) or stabilization of this sphere by a hydrogen-binding anion (F −). Spectral splitting in the 21460 cm −1 regime for transitions to the fivefold degenerate 5D 2 state determines the local symmetry of the Eu 3+ ion. Fourfold splitting is observed for Eu 3+ with a fully aquated inner hydration sphere indicating several symmetries compatible with eight or nine water molecules in the inner hydration sphere. Inner sphere NO − 3 anion reduces the hydration sphere symmetry. The data suggest a model where water molecules are mobile within the inner hydration sphere to produce a broadened bandwidth and a statistical distribution of local symmetries. Anions in the inner hydration sphere limit this mobility.