Local hydrogen-bonding structure in liquid water probed by IR-multiphoton excitation

A liquid beam of water was irradiated with an IR-laser resonant to the OH stretching mode of liquid water. Hydrated hydonium ions, H3O+(H2O)n, and hydroxyl ions, OH−(H2O)n, were ejected to the gas phase. Dependences of the ion intensity on the power and the frequency of the IR laser show that an ion-pair state, (H+⋯OH−), was produced by vibrational four-photon excitation of a water molecule in the liquid water and was subsequently separately ejected to the gas phase. The four-photon excitation proceeds in a particular local structure, where the water molecule weakly interacts with the neighboring ones.