Revealing excess protons in the infrared spectrum of liquid water

Abstract

AbstractThe most common species in liquid water, next to neutral $$\hbox {H}_2\hbox {O}$$H2O molecules, are the $$\hbox {H}_3\hbox {O}^+$$H3O+ and $$\hbox {OH}^-$$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 $$\hbox {H}_3\hbox {O}^+$$H3O+, $$\hbox {DH}_2\hbox {O}^+$$DH2O+, $$\hbox {HD}_2\hbox {O}^+$$HD2O+, and $$\hbox {D}_3\hbox {O}^+$$D3O+ ions in the IR spectra of light ($$\hbox {H}_2\hbox {O}$$H2O), heavy ($$\hbox {D}_2\hbox {O}$$D2O), and semi-heavy (HDO) water. We find that short-living ions, with concentrations reaching $$\sim 2\%$$∼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.