Elucidating low-frequency vibrational dynamics in calcite and water with time-resolved third-harmonic generation spectroscopy

Abstract

Low-frequency vibrations are foundational for material properties including thermal conductivity and chemical reactivity. To resolve the intrinsic molecular conformational dynamics in condensed phase, we implement time-resolved third-harmonic generation (TRTHG) spectroscopy to unravel collective skeletal motions in calcite, water, and aqueous salt solution in situ. The lifetime of three Raman-active modes in polycrystalline calcite at 155, 282 and 703 cm⁻¹ is found to be ca. 1.6 ps, 1.3 ps and 250 fs, respectively. The lifetime difference is due to crystallographic defects and anharmonic effects. By incorporating a home-built wire-guided liquid jet, we apply TRTHG to investigate pure water and ZnCl₂ aqueous solution, revealing ultrafast dynamics of water intermolecular stretching and librational bands below 500 cm⁻¹ and a characteristic 280 cm⁻¹ vibrational mode in the ZnCl₄(H₂O)₂²⁻ complex. TRTHG proves to be a compact and versatile technique that directly uses the 800 nm fundamental laser pulse output to capture ultrafast low-frequency vibrational motion snapshots in condensed-phase materials including the omnipresent water, which provides the important time dimension to spectral characterization of molecular structure–function relationships.