Revealing excess protons in the infrared spectrum of liquid water

TL;DR

This study uses spectral-weight analysis to detect and quantify short-lived H3O+ and related ions in liquid water, revealing their coexistence with longer-lived ions and highlighting water's ionic nature at ultrafast timescales.

Contribution

It experimentally resolves elusive short-lived ions in water's IR spectrum, providing new insights into water's ionic fluctuations at femtosecond timescales.

Findings

Short-lived ions constitute about 2% of water molecules.

Ions coexist with long-living pH-active ions on picosecond timescales.

Liquid water acts as an effective ionic liquid in femtochemistry.

Abstract

The most common species in liquid water, next to neutral H$_2$O molecules, are the H$_3$O$^+$ and OH$^-$ ions. In a dynamic picture, their exact concentrations depend on the time scale at which these are probed. Here, using a spectral-weight analysis, we experimentally resolve the fingerprints of the elusive fluctuations-born short-living H$_3$O$^+$, DH$_2$O$^+$, HD$_2$O$^+$, and D$_3$O$^+$ ions in the IR spectra of light (H$_2$O), heavy (D$_2$O), and semi-heavy (HDO) water. We find that short-living ions, with concentrations reaching $\sim 2\%$ of the content of water molecules, coexist with long-living pH-active ions on the picosecond timescale, thus making liquid water an effective ionic liquid in femtochemistry.