Stimulated Raman scattering and enhancement mechanisms in alkali metal hydroxide aqueous solutions under laser-induced dynamic pressure.

Abstract

Stimulated Raman scattering (SRS) in lithium hydroxide-water/heavy water (LiOH-H₂O/D₂O) solutions with varying concentrations was investigated under laser-induced high-pressure conditions using an Nd: YAG laser. The spectra revealed a significant enhancement in SRS signals, characterized by the emergence of low-wavenumber Raman peaks and a shift of the main SRS peak of liquid water to lower frequencies, evolving from a single peak to two or three peaks, which suggested the formation of an ice-like structure. Additionally, the normalized SRS intensity was higher than that of pure water. The enhancement of SRS signals during LiOH dissolution was attributed to the cooperative modulation between hydrogen bonding (HB, O-H:O) by OH^- and Coulombic electronic interactions generated by monovalent Li⁺, which strengthened HB interactions, increased the Raman gain coefficient, and induced structural modifications akin to pressure effects. Besides, the increased density amplified the dynamic high-pressure effects during the laser-induced breakdown (LIB) process, further boosting the SRS signals. This work provides valuable insights into the interaction mechanisms between alkali metal hydroxides and H₂O/D₂O under laser-induced pressure conditions, offering a comprehensive approach to understanding and enhancing the SRS effect in aqueous solutions.