Tuning laser-induced optical breakdown and cavitation through the ionic environment in aqueous media

TL;DR

This study investigates how ionic strength and ion type influence laser-induced cavitation in water, revealing that ion-specific chemistry affects breakdown thresholds and cavitation behavior through electron chemistry mechanisms.

Contribution

It provides the first experimental analysis of how ionic strength and ion specificity modulate optical breakdown and cavitation in aqueous electrolytes, highlighting the role of electron chemistry.

Findings

Increasing ionic strength lowers cavitation threshold universally.

Acidic solutions inhibit cavitation, alkaline solutions enhance it.

Electron chemistry, not conductivity, governs cavitation onset.

Abstract

Laser-induced cavitation in liquids originates from optical breakdown processes that depend sensitively on both laser-plasma dynamics and the chemical microenvironment of the solvent. Herein, we experimentally decouple the effects of ionic strength and ion specificity on cavitation inception in aqueous electrolytes spanning neutral, acidic, and alkaline regimes. Using focused nanosecond laser pulses, we show that increasing ionic strength universally lowers the cavitation threshold by enhancing charge screening and seed-electron availability. However, under constant ionic strength, strongly asymmetric behavior emerges: acidic (hydrogen chloride, HCl) solutions inhibit cavitation, whereas alkaline (sodium hydroxide, NaOH) solutions enhance it. This asymmetry arises from hydrated-electron kinetics that depends on the ion specificity. In acidic solutions, hydronium ions act as diffusion-limited scavengers of hydrated electrons, quenching their lifetime and inhibiting avalanche ionization. In contrast, hydroxide ions reduce hydronium availability and extend electron survival, promoting more efficient plasma formation. Despite large discrepancies in breakdown thresholds, electrical conductivity remains nearly constant, demonstrating that microscopic electron chemistry, rather than bulk charge transport, governs cavitation onset. These results establish a mechanism connection between electrolyte chemistry and optical breakdown, showing that ion-specific reaction dynamics fundamentally control laser-induced cavitation in aqueous environments.